import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.io.OutputStream;
import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.HashMap;
import java.util.Map;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.math.BigDecimal;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }

    static class Task {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int A = in.nextInt();
            int B = in.nextInt();
            int C = in.nextInt();
            int D = in.nextInt();

            boolean N[] = new boolean[101];
            boolean M[] = new boolean[101];

            for(int i=A; i<=B; i++){
                N[i] = true;
            }
            for(int i=C; i<=D; i++){
                M[i] = true;
            }
            int res=0;
            for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            }
            if(res>0) res--;
            out.println(res);

        }
    }


    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

    }
} import java.io.ByteArrayInputStream; ByteArrayInputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Arrays; Arrays java.util import java.util.InputMismatchException; InputMismatchException java.util import java.io.OutputStream; OutputStream java.io import java.util.StringTokenizer; StringTokenizer java.util import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.HashMap; HashMap java.util import java.util.Map; Map java.util import java.util.ArrayList; ArrayList java.util import java.util.List; List java.util import java.util.Scanner; Scanner java.util import java.math.BigDecimal; BigDecimal java.math public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }

    static class Task {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int A = in.nextInt();
            int B = in.nextInt();
            int C = in.nextInt();
            int D = in.nextInt();

            boolean N[] = new boolean[101];
            boolean M[] = new boolean[101];

            for(int i=A; i<=B; i++){
                N[i] = true;
            }
            for(int i=C; i<=D; i++){
                M[i] = true;
            }
            int res=0;
            for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            }
            if(res>0) res--;
            out.println(res);

        }
    }


    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out InputReader in = new InputReader(inputStream); in new InputReader(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) Task solver = new Task(); solver new Task() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class Task {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int A = in.nextInt();
            int B = in.nextInt();
            int C = in.nextInt();
            int D = in.nextInt();

            boolean N[] = new boolean[101];
            boolean M[] = new boolean[101];

            for(int i=A; i<=B; i++){
                N[i] = true;
            }
            for(int i=C; i<=D; i++){
                M[i] = true;
            }
            int res=0;
            for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            }
            if(res>0) res--;
            out.println(res);

        }
    } Task public void solve(int testNumber, InputReader in, PrintWriter out) {
            int A = in.nextInt();
            int B = in.nextInt();
            int C = in.nextInt();
            int D = in.nextInt();

            boolean N[] = new boolean[101];
            boolean M[] = new boolean[101];

            for(int i=A; i<=B; i++){
                N[i] = true;
            }
            for(int i=C; i<=D; i++){
                M[i] = true;
            }
            int res=0;
            for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            }
            if(res>0) res--;
            out.println(res);

        } solve {
            int A = in.nextInt();
            int B = in.nextInt();
            int C = in.nextInt();
            int D = in.nextInt();

            boolean N[] = new boolean[101];
            boolean M[] = new boolean[101];

            for(int i=A; i<=B; i++){
                N[i] = true;
            }
            for(int i=C; i<=D; i++){
                M[i] = true;
            }
            int res=0;
            for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            }
            if(res>0) res--;
            out.println(res);

        } int A = in.nextInt(); A in.nextInt() in.nextInt in int B = in.nextInt(); B in.nextInt() in.nextInt in int C = in.nextInt(); C in.nextInt() in.nextInt in int D = in.nextInt(); D in.nextInt() in.nextInt in boolean N[] = new boolean[101]; N new boolean[101] 101 boolean M[] = new boolean[101]; M new boolean[101] 101 for(int i=A; i<=B; i++){
                N[i] = true;
            } int i=A; int i=A; i A i<=B i B i++ i++ i {
                N[i] = true;
            } {
                N[i] = true;
            } N[i] = true N[i] N i true for(int i=C; i<=D; i++){
                M[i] = true;
            } int i=C; int i=C; i C i<=D i D i++ i++ i {
                M[i] = true;
            } {
                M[i] = true;
            } M[i] = true M[i] M i true int res=0; res 0 for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            } int i=0; int i=0; i 0 i<=100 i 100 i++ i++ i {
                if(N[i]==true && M[i]==true){
                    res++;
                }
            } {
                if(N[i]==true && M[i]==true){
                    res++;
                }
            } if(N[i]==true && M[i]==true){
                    res++;
                } N[i]==true && M[i]==true N[i]==true N[i] N i true M[i]==true M[i] M i true {
                    res++;
                } res++ res if(res>0) res--; res>0 res 0 res-- res out.println(res) out.println out res int testNumber testNumber InputReader in in PrintWriter out out static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

    } InputReader public BufferedReader reader; reader public StringTokenizer tokenizer; tokenizer public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        } InputReader {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        } reader = new BufferedReader(new InputStreamReader(stream), 32768) reader new BufferedReader(new InputStreamReader(stream), 32768) tokenizer = null tokenizer null InputStream stream stream public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } next {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            } tokenizer == null || !tokenizer.hasMoreTokens() tokenizer == null tokenizer null !tokenizer.hasMoreTokens() tokenizer.hasMoreTokens() tokenizer.hasMoreTokens tokenizer {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            } try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } IOException e {
                    throw new RuntimeException(e);
                } throw new RuntimeException(e); new RuntimeException(e) {
                    tokenizer = new StringTokenizer(reader.readLine());
                } tokenizer = new StringTokenizer(reader.readLine()) tokenizer new StringTokenizer(reader.readLine()) return tokenizer.nextToken(); tokenizer.nextToken() tokenizer.nextToken tokenizer public int nextInt() {
            return Integer.parseInt(next());
        } nextInt {
            return Integer.parseInt(next());
        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next public long nextLong() {
            return Long.parseLong(next());
        } nextLong {
            return Long.parseLong(next());
        } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }

    static class Task {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int A = in.nextInt();
            int B = in.nextInt();
            int C = in.nextInt();
            int D = in.nextInt();

            boolean N[] = new boolean[101];
            boolean M[] = new boolean[101];

            for(int i=A; i<=B; i++){
                N[i] = true;
            }
            for(int i=C; i<=D; i++){
                M[i] = true;
            }
            int res=0;
            for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            }
            if(res>0) res--;
            out.println(res);

        }
    }


    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

    }
} public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }

    static class Task {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int A = in.nextInt();
            int B = in.nextInt();
            int C = in.nextInt();
            int D = in.nextInt();

            boolean N[] = new boolean[101];
            boolean M[] = new boolean[101];

            for(int i=A; i<=B; i++){
                N[i] = true;
            }
            for(int i=C; i<=D; i++){
                M[i] = true;
            }
            int res=0;
            for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            }
            if(res>0) res--;
            out.println(res);

        }
    }


    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

    }
} Main
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.io.OutputStream;
import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.HashMap;
import java.util.Map;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.math.BigDecimal;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }

    static class Task {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int A = in.nextInt();
            int B = in.nextInt();
            int C = in.nextInt();
            int D = in.nextInt();

            boolean N[] = new boolean[101];
            boolean M[] = new boolean[101];

            for(int i=A; i<=B; i++){
                N[i] = true;
            }
            for(int i=C; i<=D; i++){
                M[i] = true;
            }
            int res=0;
            for(int i=0; i<=100; i++){
                if(N[i]==true && M[i]==true){
                    res++;
                }
            }
            if(res>0) res--;
            out.println(res);

        }
    }


    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

    }
}