import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Arrays;

public class Main {

    public static void main(String[] args) throws Exception {
        try (Reader r = new Reader(System.in)) {
            int N = r.i();
            int M = r.i();

            int[][] LRC = r.im(M);
            Arrays.sort(LRC, (o1, o2) -> {
                int compare = Integer.compare(o1[0], o2[0]);
                if (compare != 0) {
                    return compare;
                }
                return Integer.compare(o1[2], o2[2]);
            });

            long[] distance = new long[N];
            for (int i = 1; i < N; i++) {
                distance[i] = Long.MAX_VALUE;
            }

            int preL = -1;
            int preR = -1;
            for (int[] lrc : LRC) {
                int L = lrc[0] - 1;
                int R = lrc[1] - 1;
                long C = lrc[2];

                if (distance[L] == Long.MAX_VALUE) {
                    System.out.println(-1);
                    return;
                }

                int t = L + 1;
                if (preL == L) {
                    if (preR >= R) {
                        continue;
                    }
                    t = preR + 1;
                }

                for (int z = R; z >= t; z--) {
                    if (distance[z] < distance[L] + C) {
                        break;
                    }
                    distance[z] = distance[L] + C;
                }

                preL = L;
                preR = R;
            }

            long d = distance[N - 1];
            System.out.println(d == Long.MAX_VALUE ? -1 : d);
        }
    }

    public static class Reader implements AutoCloseable {

        private BufferedReader br;

        private String[] buf;

        private int bufidx = -1;

        public Reader(InputStream in) {
            this.br = new BufferedReader(new InputStreamReader(in));
        }

        @Override
        public void close() throws Exception {
            br.close();
        }

        private void readBuf() throws IOException {
            if (bufidx == -1 || bufidx == buf.length) {
                buf = br.readLine().split(" ");
                bufidx = 0;
            }
        }

        public String s() throws IOException {
            readBuf();
            return buf[bufidx++];
        }

        public int i() throws IOException {
            return Integer.parseInt(s());
        }

        public long l() throws IOException {
            return Long.parseLong(s());
        }

        public double d() throws IOException {
            return Double.parseDouble(s());
        }

        public String[] sa() throws IOException {
            readBuf();
            if (bufidx != 0) {
                throw new RuntimeException("illegal read");
            }
            bufidx = -1;
            return buf;
        }

        public int[] ia() throws IOException {
            String[] sa = sa();
            int[] ia = new int[sa.length];
            for (int i = 0; i < sa.length; i++) {
                ia[i] = Integer.parseInt(sa[i]);
            }
            return ia;
        }

        public long[] la() throws IOException {
            String[] sa = sa();
            long[] la = new long[sa.length];
            for (int i = 0; i < sa.length; i++) {
                la[i] = Long.parseLong(sa[i]);
            }
            return la;
        }

        public double[] da() throws IOException {
            String[] sa = sa();
            double[] da = new double[sa.length];
            for (int i = 0; i < sa.length; i++) {
                da[i] = Double.parseDouble(sa[i]);
            }
            return da;
        }

        public int[][] im(int N) throws IOException {
            int[][] im = new int[N][];
            for (int i = 0; i < N; i++) {
                im[i] = ia();
            }
            return im;
        }

        public long[][] lm(int N) throws IOException {
            long[][] lm = new long[N][];
            for (int i = 0; i < N; i++) {
                lm[i] = la();
            }
            return lm;
        }

        public double[][] dm(int N) throws IOException {
            double[][] dm = new double[N][];
            for (int i = 0; i < N; i++) {
                dm[i] = da();
            }
            return dm;
        }

    }

}


Measured memory_bytes: 50252