import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.NoSuchElementException;
import java.util.function.IntUnaryOperator;
import java.util.function.LongBinaryOperator;
import java.util.function.LongUnaryOperator;


public class Main {
    public static void main(String[] args) throws Exception {
        Field f = System.out.getClass().getDeclaredField("autoFlush");
        f.setAccessible(true);
        f.set(System.out, false);
        execute(System.in, System.out);
    }

    public static void execute(InputStream in, OutputStream out) {
        ExtendedScanner s = new ExtendedScanner(in);
        Out o = new Out(out);
        solve(s, o);
        o.flush();
        o.close();
    }

    public static void solve(ExtendedScanner sc, Out out) {
        int n = sc.nextInt();
        int m = sc.nextInt();
        long[] dp = new long[n];
        Arrays.fill(dp, 1, n, Infty.LONG);
        int[] l = new int[m];
        int[] r = new int[m];
        long[] c = new long[m];
        for (int i = 0; i < m; i++) {
            l[i] = sc.nextInt() - 1;
            r[i] = sc.nextInt() - 1;
            c[i] = sc.nextLong();
        }
        int[] p = IndexSort.sort(r);
        LongSegmentTree t = new LongSegmentTree(n, Infty.LONG, Long::min);
        t.build(dp);
        for (int i = 0; i < m; i++) {
            int rp = r[p[i]];
            int lp = l[p[i]];
            long cp = c[p[i]];
            long min = Math.min(t.query(lp, rp) + cp, t.get(rp));
            t.update(rp, v -> min);
        }
        long ans = t.get(n - 1);
        out.writeln(ans == Infty.LONG ? -1 : ans);
    }
}


class LongSegmentTree {
    private final long[] dat;
    private final int n;
    private final long e;
    private final LongBinaryOperator f;
    public LongSegmentTree(final int n, final long e, final LongBinaryOperator f) {
        this.e = e;
        this.f = f;
        int k = 1; while (k < n) k <<= 1;
        this.n = k;
        this.dat = new long[k << 1];
        clear();
    }
    public void build(final long[] a) {
        System.arraycopy(a, 0, dat, n, a.length);
        for (int i = n - 1; i > 0; i--) dat[i] = f.applyAsLong(dat[i << 1 | 0], dat[i << 1 | 1]);
    }
    public void update(int i, final LongUnaryOperator g) {
        i += n;
        dat[i] = g.applyAsLong(dat[i]);
        for (i >>= 1; i > 0; i >>= 1) dat[i] = f.applyAsLong(dat[i << 1 | 0], dat[i << 1 | 1]);
    }
    public long get(int i) {
        return dat[i + n];
    }
    public long query(int l, int r) {
        long resL = e, resR = e;
        for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
            if ((l & 1) != 0) resL = f.applyAsLong(resL, dat[l++]);
            if ((r & 1) != 0) resR = f.applyAsLong(dat[--r], resR);
        }
        return f.applyAsLong(resL, resR);
    }
    public void clear() {
        Arrays.fill(dat, e);
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 */
class BasicScanner {
    private final InputStream in;
    private final byte[] buf = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    public BasicScanner(InputStream in) {this.in = in;}
    private boolean hasNextByte() {
        if (ptr < buflen) return true;
        ptr = 0;
        try {buflen = in.read(buf);}
        catch (final IOException e) {e.printStackTrace();}
        return buflen > 0;
    }
    private int readByte() {return hasNextByte() ? buf[ptr++] : -1;}
    public boolean hasNext() {
        while (hasNextByte() && !(33 <= buf[ptr] && buf[ptr] <= 126)) ptr++;
        return hasNextByte();
    }
    private StringBuilder nextSequence() {
        if (!hasNext()) throw new NoSuchElementException();
        final StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (33 <= b && b <= 126) {sb.appendCodePoint(b); b = readByte();}
        return sb;
    }
    public char nextChar() {
        return (char) readByte();
    }
    public String next() {
        return nextSequence().toString();
    }
    public String next(int len) {
        return new String(nextChars(len));
    }
    public char[] nextChars() {
        final StringBuilder sb = nextSequence();
        int l = sb.length();
        char[] dst = new char[l];
        sb.getChars(0, l, dst, 0);
        return dst;
    }
    public char[] nextChars(int len) {
        if (!hasNext()) throw new NoSuchElementException();
        char[] s = new char[len];
        int i = 0;
        int b = readByte();
        while (33 <= b && b <= 126 && i < len) {s[i++] = (char) b; b = readByte();}
        if (i != len) throw new NoSuchElementException(String.format("length %d is longer than the sequence.", len));
        return s;
    }
    public 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 = n * 10 + b - '0';
            else if (b == -1 || !(33 <= b && b <= 126)) return minus ? -n : n;
            else throw new NumberFormatException();
            b = readByte();
        }
    }
    public int nextInt() {return Math.toIntExact(nextLong());}
    public double nextDouble() {return Double.parseDouble(next());}
}


/**
 * @author https://atcoder.jp/users/suisen
 */
class Out {
    private final OutputStream out;
    private byte[] buf = new byte[1024];
    private int ptr = 0;
    private static final int AUTO_FLUSH_THRETHOLD = 1 << 17;

    public Out(OutputStream out) {
        this.out = out;
    }

    public void flush() {
        try {
            out.write(buf, 0, ptr);
            out.flush();
            ptr = 0;
        } catch (IOException e) {e.printStackTrace();}
    }

    public void close() {
        try {out.close();} catch (IOException e) {e.printStackTrace();}
    }

    public Out writeln() {return write('\n');}
    public Out writeln(Object o) {return write(o).write('\n');}
    public Out writeln(String s) {return write(s).write('\n');}
    public Out writeln(char[] c) {return write(c).write('\n');}
    public Out writeln(char   c) {return write(c).write('\n');}
    public Out writeln(byte   b) {return write(b).write('\n');}
    public Out writeln(int    x) {return write(x).write('\n');}
    public Out writeln(long   x) {return write(x).write('\n');}
    public Out writeln(double d) {return write(d).write('\n');}

    public Out writeSpace() {return write(' ');}
    
    public Out write(Object o) {
        return write(o.toString());
    }

    public Out write(String s) {
        try {
            Field strValueField = s.getClass().getDeclaredField("value");
            strValueField.setAccessible(true);
            byte[] b = (byte[]) strValueField.get(s);
            int l = s.length();
            if (l > AUTO_FLUSH_THRETHOLD) {
                flush();
                int i = 0;
                while (i + AUTO_FLUSH_THRETHOLD < l) {
                    out.write(b, i, AUTO_FLUSH_THRETHOLD);
                    out.flush();
                    i += AUTO_FLUSH_THRETHOLD;
                }
                ensureCapacity(l - i);
                System.arraycopy(b, i, buf, 0, l - i);
                ptr = l - i;
            } else {
                ensureCapacity(ptr + l);
                System.arraycopy(b, 0, buf, ptr, l);
                ptr += l;
            }
        } catch (Exception e) {e.printStackTrace();}
        return this;
    }

    public Out write(char[] c) {
        int l = c.length;
        if (l > AUTO_FLUSH_THRETHOLD) {
            flush();
            ensureCapacity(AUTO_FLUSH_THRETHOLD);
            int i = 0;
            while (i + AUTO_FLUSH_THRETHOLD < l) {
                for (int j = 0; j < AUTO_FLUSH_THRETHOLD; j++) {
                    buf[ptr++] = (byte) c[i + j];
                }
                flush();
                i += AUTO_FLUSH_THRETHOLD;
            }
            for (; i < l; i++) {
                buf[ptr++] = (byte) c[i];
            }
        } else {
            ensureCapacity(ptr + l);
            for (char ch : c) buf[ptr++] = (byte) ch;
        }
        return this;
    }

    public Out write(char c) {
        ensureCapacity(ptr + 1);
        buf[ptr++] = (byte) c;
        return this;
    }

    public Out write(byte b) {
        ensureCapacity(ptr + 1);
        buf[ptr++] = b;
        return this;
    }

    public Out write(int x) {
        if (x == 0) {
            ensureCapacity(ptr + 1);
            buf[ptr++] = '0';
            return this;
        }
        int d = stringSize(x);
        ensureCapacity(ptr + d);
        if (x < 0) {
            buf[ptr++] = '-';
            x = -x;
            d--;
        }
        int j = ptr + d; 
        while (x > 0) {
            buf[--j] = (byte) ('0' + (x % 10));
            x /= 10;
        }
        ptr += d;
        return this;
    }

    public Out write(long x) {
        if (x == 0) {
            ensureCapacity(ptr + 1);
            buf[ptr++] = '0';
            return this;
        }
        int d = stringSize(x);
        ensureCapacity(ptr + d);
        if (x < 0) {
            buf[ptr++] = '-';
            x = -x;
            d--;
        }
        int j = ptr + d; 
        while (x > 0) {
            buf[--j] = (byte) ('0' + (x % 10));
            x /= 10;
        }
        ptr += d;
        return this;
    }

    public Out write(double d) {
        return write(Double.toString(d));
    }

    private void ensureCapacity(int cap) {
        if (cap > AUTO_FLUSH_THRETHOLD) {
            flush();
        }
        if (cap >= buf.length) {
            int newLength = buf.length;
            while (newLength < cap) newLength <<= 1;
            byte[] newBuf = new byte[newLength];
            System.arraycopy(buf, 0, newBuf, 0, buf.length);
            buf = newBuf;
        }
    }
    private static int stringSize(long x) {
        int d = 1;
        if (x >= 0) {d = 0; x = -x;}
        long p = -10;
        for (int i = 1; i < 19; i++, p *= 10) if (x > p) return i + d;
        return 19 + d;
    }
    private static int stringSize(int x) {
        int d = 1;
        if (x >= 0) {d = 0; x = -x;}
        int p = -10;
        for (int i = 1; i < 10; i++, p *= 10) if (x > p) return i + d;
        return 10 + d;
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 */
final class ExtendedScanner extends BasicScanner {
    public ExtendedScanner(InputStream in) {super(in);}
    public int[] ints(final int n) {
        final int[] a = new int[n];
        Arrays.setAll(a, $ -> nextInt());
        return a;
    }
    public int[] ints(final int n, final IntUnaryOperator f) {
        final int[] a = new int[n];
        Arrays.setAll(a, $ -> f.applyAsInt(nextInt()));
        return a;
    }
    public int[][] ints(final int n, final int m) {
        final int[][] a = new int[n][];
        Arrays.setAll(a, $ -> ints(m));
        return a;
    }
    public int[][] ints(final int n, final int m, final IntUnaryOperator f) {
        final int[][] a = new int[n][];
        Arrays.setAll(a, $ -> ints(m, f));
        return a;
    }
    public long[] longs(final int n) {
        final long[] a = new long[n];
        Arrays.setAll(a, $ -> nextLong());
        return a;
    }
    public long[] longs(final int n, final LongUnaryOperator f) {
        final long[] a = new long[n];
        Arrays.setAll(a, $ -> f.applyAsLong(nextLong()));
        return a;
    }
    public long[][] longs(final int n, final int m) {
        final long[][] a = new long[n][];
        Arrays.setAll(a, $ -> longs(m));
        return a;
    }
    public long[][] longs(final int n, final int m, final LongUnaryOperator f) {
        final long[][] a = new long[n][];
        Arrays.setAll(a, $ -> longs(m, f));
        return a;
    }
    public char[][] charArrays(final int n) {
        final char[][] c = new char[n][];
        Arrays.setAll(c, $ -> nextChars());
        return c;
    }
    public char[][] charArrays(final int n, final int m) {
        final char[][] c = new char[n][];
        Arrays.setAll(c, $ -> nextChars(m));
        return c;
    }
    public double[] doubles(final int n) {
        final double[] a = new double[n];
        Arrays.setAll(a, $ -> nextDouble());
        return a;
    }
    public double[][] doubles(final int n, final int m) {
        final double[][] a = new double[n][];
        Arrays.setAll(a, $ -> doubles(m));
        return a;
    }
    public String[] strings(final int n) {
        final String[] s = new String[n];
        Arrays.setAll(s, $ -> next());
        return s;
    }
    public String[] strings(final int n, final int m) {
        final String[] s = new String[n];
        Arrays.setAll(s, $ -> next(m));
        return s;
    }
}

final class Infty {
    public static final long   LONG   = 1l << 59;
    public static final int    INT    = 1  << 30;
    public static final double DOUBLE = 1e150;
    private Infty(){}
}

@FunctionalInterface
interface IntComparator {
    public int compare(int i, int j);
    public default boolean eq(final int i, final int j) {return compare(i, j) == 0;}
    public default boolean ne(final int i, final int j) {return compare(i, j) != 0;}
    public default boolean gt(final int i, final int j) {return compare(i, j) > 0;}
    public default boolean ge(final int i, final int j) {return compare(i, j) >= 0;}
    public default boolean lt(final int i, final int j) {return compare(i, j) < 0;}
    public default boolean le(final int i, final int j) {return compare(i, j) <= 0;}
}


final class IndexSort {
    public static int[] sort(final int[] a) {
        final int[] index = new int[a.length];
        Arrays.setAll(index, IntUnaryOperator.identity());
        ComparativeMergeSort.sort(index, (i, j) -> a[i] - a[j]);
        return index;
    }
    public static int[] sortDescending(final int[] a) {
        final int[] index = new int[a.length];
        Arrays.setAll(index, IntUnaryOperator.identity());
        ComparativeMergeSort.sort(index, (i, j) -> a[j] - a[i]);
        return index;
    }
    public static int[] sort(final long[] a) {
        final int[] index = new int[a.length];
        Arrays.setAll(index, IntUnaryOperator.identity());
        ComparativeMergeSort.sort(index, (i, j) -> Long.compare(a[i], a[j]));
        return index;
    }
    public static int[] sortDescending(final long[] a) {
        final int[] index = new int[a.length];
        Arrays.setAll(index, IntUnaryOperator.identity());
        ComparativeMergeSort.sort(index, (i, j) -> Long.compare(a[j], a[i]));
        return index;
    }
}

@FunctionalInterface
interface LongComparator {
    public int compare(long i, long j);
    public default boolean eq(final long i, final long j) {return compare(i, j) == 0;}
    public default boolean ne(final long i, final long j) {return compare(i, j) != 0;}
    public default boolean gt(final long i, final long j) {return compare(i, j) > 0;}
    public default boolean ge(final long i, final long j) {return compare(i, j) >= 0;}
    public default boolean lt(final long i, final long j) {return compare(i, j) < 0;}
    public default boolean le(final long i, final long j) {return compare(i, j) <= 0;}
}


final class ComparativeMergeSort {
    private static int INSERTION_SORT_THRESHOLD = 60;
    public static void sort(final int[] a, final IntComparator comparator) {sort(a, 0, a.length, comparator);}
    public static void sort(final int[] a, final int begin, final int end, final IntComparator comparator) {
        for (int i = begin;;) {
            final int j = i + INSERTION_SORT_THRESHOLD;
            if (j < end) ComparativeInsertionSort.sort(a, i, j, comparator);
            else {ComparativeInsertionSort.sort(a, i, end, comparator); break;}
            i = j;
        }
        final int len = end - begin;
        final int[] work = new int[len];
        for (int block = INSERTION_SORT_THRESHOLD; block <= len; block <<= 1) {
            final int twoBlocks = block << 1;
            for (int from = begin, max = end - block; from < max; from += twoBlocks) {
                final int mid = from + block, to = Math.min(from + twoBlocks, end);
                System.arraycopy(a, from, work, 0, block);
                for (int i = from, wi = 0, ti = mid;; i++) {
                    if (ti == to) {System.arraycopy(work, wi, a, i, block - wi); break;}
                    if (comparator.gt(work[wi], a[ti])) a[i] = a[ti++];
                    else {a[i] = work[wi++]; if (wi == block) break;}
                }
            }
        }
    }
    public static void sort(final long[] a, final LongComparator comparator) {sort(a, 0, a.length, comparator);}
    public static void sort(final long[] a, final int begin, final int end, final LongComparator comparator) {
        for (int i = begin;;) {
            final int j = i + INSERTION_SORT_THRESHOLD;
            if (j < end) ComparativeInsertionSort.sort(a, i, j, comparator);
            else {ComparativeInsertionSort.sort(a, i, end, comparator); break;}
            i = j;
        }
        final int len = end - begin;
        final long[] work = new long[len];
        for (int block = INSERTION_SORT_THRESHOLD; block <= len; block <<= 1) {
            final int twoBlocks = block << 1;
            for (int from = begin, max = end - block; from < max; from += twoBlocks) {
                final int mid = from + block, to = Math.min(from + twoBlocks, end);
                System.arraycopy(a, from, work, 0, block);
                for (int i = from, wi = 0, ti = mid;; i++) {
                    if (ti == to) {System.arraycopy(work, wi, a, i, block - wi); break;}
                    if (comparator.gt(work[wi], a[ti])) a[i] = a[ti++];
                    else {a[i] = work[wi++]; if (wi == block) break;}
                }
            }
        }
    }
}


final class ComparativeInsertionSort {
    static void sort(final int[] a, final int from, final int to, final IntComparator comparator) {
        for (int i = from + 1; i < to; i++) {
            final int tmp = a[i];
            if (comparator.gt(a[i - 1], tmp)) {
                int j = i;
                do {a[j] = a[j - 1]; j--;} while (j > from && comparator.gt(a[j - 1], tmp));
                a[j] = tmp;
            }
        }
    }
    static void sort(final long[] a, final int from, final int to, final LongComparator comparator) {
        for (int i = from + 1; i < to; i++) {
            final long tmp = a[i];
            if (comparator.gt(a[i - 1], tmp)) {
                int j = i;
                do {a[j] = a[j - 1]; j--;} while (j > from && comparator.gt(a[j - 1], tmp));
                a[j] = tmp;
            }
        }
    }
}
