import java.io.IOException;
import java.util.NoSuchElementException;

import java.io.InputStream;

import java.io.PrintWriter;

import java.util.Arrays;

@SuppressWarnings("unchecked")
public class Main {
    public static void main(String[] args) throws IOException {
        PrintWriter out = new PrintWriter(System.out);

        final int N = nextInt();

        int[] h = new int[N];
        int[] after = new int[N];

        for (int i = 0; i < N; i++) {
            h[i] = nextInt();
        }

        boolean[] done = new boolean[N];

        int cnt = 0;

        for (int i = 0; i < N; i++) {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        }

        boolean flg = true;

        int ans = 0;

        while (flg) {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        }

        out.println(ans);

        out.flush();
    }

    // FastScanner start
    static final InputStream in = System.in;
    static final byte[] buffer = new byte[1024];
    static int ptr = 0;
    static int buflen = 0;
    static boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    static int readByte() {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    }
    static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }
    static boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    }
    static String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    static long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    static int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    }
    static double nextDouble() {
        return Double.parseDouble(next());
    }
    // FastScanner end
} import java.io.IOException; IOException java.io import java.util.NoSuchElementException; NoSuchElementException java.util import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Arrays; Arrays java.util public class Main {
    public static void main(String[] args) throws IOException {
        PrintWriter out = new PrintWriter(System.out);

        final int N = nextInt();

        int[] h = new int[N];
        int[] after = new int[N];

        for (int i = 0; i < N; i++) {
            h[i] = nextInt();
        }

        boolean[] done = new boolean[N];

        int cnt = 0;

        for (int i = 0; i < N; i++) {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        }

        boolean flg = true;

        int ans = 0;

        while (flg) {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        }

        out.println(ans);

        out.flush();
    }

    // FastScanner start
    static final InputStream in = System.in;
    static final byte[] buffer = new byte[1024];
    static int ptr = 0;
    static int buflen = 0;
    static boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    static int readByte() {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    }
    static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }
    static boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    }
    static String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    static long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    static int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    }
    static double nextDouble() {
        return Double.parseDouble(next());
    }
    // FastScanner end
} Main // FastScanner start
    static final InputStream in = System.in; in System.in System System.in static final byte[] buffer = new byte[1024]; buffer new byte[1024] 1024 static int ptr = 0; ptr 0 static int buflen = 0; buflen 0 public static void main(String[] args) throws IOException {
        PrintWriter out = new PrintWriter(System.out);

        final int N = nextInt();

        int[] h = new int[N];
        int[] after = new int[N];

        for (int i = 0; i < N; i++) {
            h[i] = nextInt();
        }

        boolean[] done = new boolean[N];

        int cnt = 0;

        for (int i = 0; i < N; i++) {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        }

        boolean flg = true;

        int ans = 0;

        while (flg) {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        }

        out.println(ans);

        out.flush();
    } main {
        PrintWriter out = new PrintWriter(System.out);

        final int N = nextInt();

        int[] h = new int[N];
        int[] after = new int[N];

        for (int i = 0; i < N; i++) {
            h[i] = nextInt();
        }

        boolean[] done = new boolean[N];

        int cnt = 0;

        for (int i = 0; i < N; i++) {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        }

        boolean flg = true;

        int ans = 0;

        while (flg) {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        }

        out.println(ans);

        out.flush();
    } PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) final int N = nextInt(); N nextInt() nextInt int[] h = new int[N]; h new int[N] N int[] after = new int[N]; after new int[N] N for (int i = 0; i < N; i++) {
            h[i] = nextInt();
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            h[i] = nextInt();
        } {
            h[i] = nextInt();
        } h[i] = nextInt() h[i] h i nextInt() nextInt boolean[] done = new boolean[N]; done new boolean[N] N int cnt = 0; cnt 0 for (int i = 0; i < N; i++) {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        } {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        } if (h[i] == 0) {
                done[i] = true;
                cnt++;
            } h[i] == 0 h[i] h i 0 {
                done[i] = true;
                cnt++;
            } done[i] = true done[i] done i true cnt++ cnt boolean flg = true; flg true int ans = 0; ans 0 while (flg) {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        } flg {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        } if (cnt == N) break; cnt == N cnt N break; ans++ ans boolean on = false; on false boolean change = false; change false for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            } {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            } if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                } !done[i] done[i] done i {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } on = true on true if (change) {
                        ans++;
                        change = false;
                    } change {
                        ans++;
                        change = false;
                    } ans++ ans change = false change false after[i]++ after[i] after i if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    } after[i] == h[i] after[i] after i h[i] h i {
                        done[i] = true;
                        cnt++;
                    } done[i] = true done[i] done i true cnt++ cnt {
                    if (on) {
                        change = true;
                        on = false;
                    }
                } if (on) {
                        change = true;
                        on = false;
                    } on {
                        change = true;
                        on = false;
                    } change = true change true on = false on false out.println(ans) out.println out ans out.flush() out.flush out String[] args args static boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    } hasNextByte {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    } if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        } ptr < buflen ptr buflen {
            return true;
        } return true; true {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        } ptr = 0 ptr 0 try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } IOException e {
                e.printStackTrace();
            } e.printStackTrace() e.printStackTrace e {
                buflen = in.read(buffer);
            } buflen = in.read(buffer) buflen in.read(buffer) in.read in buffer if (buflen <= 0) {
                return false;
            } buflen <= 0 buflen 0 {
                return false;
            } return false; false return true; true static int readByte() {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    } readByte {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    } if (hasNextByte()) return buffer[ptr++];
        else return -1; hasNextByte() hasNextByte return buffer[ptr++]; buffer[ptr++] buffer ptr++ ptr return -1; -1 1 static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    } isPrintableChar {
        return 33 <= c && c <= 126;
    } return 33 <= c && c <= 126; 33 <= c && c <= 126 33 <= c 33 c c <= 126 c 126 int c c static boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    } hasNext {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    } while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; hasNextByte() && !isPrintableChar(buffer[ptr]) hasNextByte() hasNextByte !isPrintableChar(buffer[ptr]) isPrintableChar(buffer[ptr]) isPrintableChar buffer[ptr] buffer ptr ptr++ ptr return hasNextByte(); hasNextByte() hasNextByte static String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    } next {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    } if (!hasNext()) throw new NoSuchElementException(); !hasNext() hasNext() hasNext throw new NoSuchElementException(); new NoSuchElementException() StringBuilder sb = new StringBuilder(); sb new StringBuilder() int b = readByte(); b readByte() readByte while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        } isPrintableChar(b) isPrintableChar b {
            sb.appendCodePoint(b);
            b = readByte();
        } sb.appendCodePoint(b) sb.appendCodePoint sb b b = readByte() b readByte() readByte return sb.toString(); sb.toString() sb.toString sb static long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    } nextLong {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    } if (!hasNext()) throw new NoSuchElementException(); !hasNext() hasNext() hasNext throw new NoSuchElementException(); new NoSuchElementException() long n = 0; n 0 boolean minus = false; minus false int b = readByte(); b readByte() readByte if (b == '-') {
            minus = true;
            b = readByte();
        } b == '-' b '-' {
            minus = true;
            b = readByte();
        } minus = true minus true b = readByte() b readByte() readByte if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        } b < '0' || '9' < b b < '0' b '0' '9' < b '9' b {
            throw new NumberFormatException();
        } throw new NumberFormatException(); new NumberFormatException() while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        } true {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        } if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            } '0' <= b && b <= '9' '0' <= b '0' b b <= '9' b '9' {
                n *= 10;
                n += b - '0';
            } n *= 10 n 10 n += b - '0' n b - '0' b '0' if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            } b == -1 || !isPrintableChar(b) b == -1 b -1 1 !isPrintableChar(b) isPrintableChar(b) isPrintableChar b {
                return minus ? -n : n;
            } return minus ? -n : n; minus ? -n : n minus -n n n {
                throw new NumberFormatException();
            } throw new NumberFormatException(); new NumberFormatException() b = readByte() b readByte() readByte static int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    } nextInt {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    } long nl = nextLong(); nl nextLong() nextLong if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException(); nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE nl < Integer.MIN_VALUE nl Integer.MIN_VALUE Integer Integer.MIN_VALUE nl > Integer.MAX_VALUE nl Integer.MAX_VALUE Integer Integer.MAX_VALUE throw new NumberFormatException(); new NumberFormatException() return (int) nl; (int) nl static double nextDouble() {
        return Double.parseDouble(next());
    } nextDouble {
        return Double.parseDouble(next());
    } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next public class Main {
    public static void main(String[] args) throws IOException {
        PrintWriter out = new PrintWriter(System.out);

        final int N = nextInt();

        int[] h = new int[N];
        int[] after = new int[N];

        for (int i = 0; i < N; i++) {
            h[i] = nextInt();
        }

        boolean[] done = new boolean[N];

        int cnt = 0;

        for (int i = 0; i < N; i++) {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        }

        boolean flg = true;

        int ans = 0;

        while (flg) {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        }

        out.println(ans);

        out.flush();
    }

    // FastScanner start
    static final InputStream in = System.in;
    static final byte[] buffer = new byte[1024];
    static int ptr = 0;
    static int buflen = 0;
    static boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    static int readByte() {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    }
    static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }
    static boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    }
    static String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    static long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    static int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    }
    static double nextDouble() {
        return Double.parseDouble(next());
    }
    // FastScanner end
} public class Main {
    public static void main(String[] args) throws IOException {
        PrintWriter out = new PrintWriter(System.out);

        final int N = nextInt();

        int[] h = new int[N];
        int[] after = new int[N];

        for (int i = 0; i < N; i++) {
            h[i] = nextInt();
        }

        boolean[] done = new boolean[N];

        int cnt = 0;

        for (int i = 0; i < N; i++) {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        }

        boolean flg = true;

        int ans = 0;

        while (flg) {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        }

        out.println(ans);

        out.flush();
    }

    // FastScanner start
    static final InputStream in = System.in;
    static final byte[] buffer = new byte[1024];
    static int ptr = 0;
    static int buflen = 0;
    static boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    static int readByte() {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    }
    static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }
    static boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    }
    static String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    static long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    static int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    }
    static double nextDouble() {
        return Double.parseDouble(next());
    }
    // FastScanner end
} Main
import java.io.IOException;
import java.util.NoSuchElementException;

import java.io.InputStream;

import java.io.PrintWriter;

import java.util.Arrays;

@SuppressWarnings("unchecked")
public class Main {
    public static void main(String[] args) throws IOException {
        PrintWriter out = new PrintWriter(System.out);

        final int N = nextInt();

        int[] h = new int[N];
        int[] after = new int[N];

        for (int i = 0; i < N; i++) {
            h[i] = nextInt();
        }

        boolean[] done = new boolean[N];

        int cnt = 0;

        for (int i = 0; i < N; i++) {
            if (h[i] == 0) {
                done[i] = true;
                cnt++;
            }
        }

        boolean flg = true;

        int ans = 0;

        while (flg) {
            if (cnt == N) break;

            ans++;

            boolean on = false;
            boolean change = false;

            for(int i = 0; i < N; i++) {
                if (!done[i]) {
                    on = true;
                    if (change) {
                        ans++;
                        change = false;
                    }
                    after[i]++;
                    if (after[i] == h[i]) {
                        done[i] = true;
                        cnt++;
                    }
                } else {
                    if (on) {
                        change = true;
                        on = false;
                    }
                }
            }
        }

        out.println(ans);

        out.flush();
    }

    // FastScanner start
    static final InputStream in = System.in;
    static final byte[] buffer = new byte[1024];
    static int ptr = 0;
    static int buflen = 0;
    static boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    static int readByte() {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    }
    static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }
    static boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    }
    static String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    static long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if(b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    static int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    }
    static double nextDouble() {
        return Double.parseDouble(next());
    }
    // FastScanner end
}
