#include <bits/stdc++.h>
using namespace std;
using Int = long long int;
template <typename T, typename E>
struct SegmentTree {
  typedef function<T(T, T)> F;
  typedef function<T(T, E)> G;
  typedef function<E(E, E)> H;
  typedef function<E(E, int)> P;
  int n;
  F f;
  G g;
  H h;
  P p;
  T d1;
  E d0;
  vector<T> dat;
  vector<E> laz;
  SegmentTree(
      int n_, F f, G g, H h, T d1, E d0, vector<T> v = vector<T>(),
      P p = [](E a, int b) { return a; })
      : f(f), g(g), h(h), d1(d1), d0(d0), p(p) {
    init(n_);
    if (n_ == (int)v.size()) build(n_, v);
  }
  void init(int n_) {
    n = 1;
    while (n < n_) n *= 2;
    dat.clear();
    dat.resize(2 * n - 1, d1);
    laz.clear();
    laz.resize(2 * n - 1, d0);
  }
  void build(int n_, vector<T> v) {
    for (int i = 0; i < n_; i++) dat[i + n - 1] = v[i];
    for (int i = n - 2; i >= 0; i--) dat[i] = f(dat[i * 2 + 1], dat[i * 2 + 2]);
  }
  inline void eval(int len, int k) {
    if (laz[k] == d0) return;
    if (k * 2 + 1 < n * 2 - 1) {
      laz[k * 2 + 1] = h(laz[k * 2 + 1], laz[k]);
      laz[k * 2 + 2] = h(laz[k * 2 + 2], laz[k]);
    }
    dat[k] = g(dat[k], p(laz[k], len));
    laz[k] = d0;
  }
  T update(int a, int b, E x, int k, int l, int r) {
    eval(r - l, k);
    if (r <= a || b <= l) return dat[k];
    if (a <= l && r <= b) {
      laz[k] = h(laz[k], x);
      return g(dat[k], p(laz[k], r - l));
    }
    return dat[k] = f(update(a, b, x, k * 2 + 1, l, (l + r) / 2),
                      update(a, b, x, k * 2 + 2, (l + r) / 2, r));
  }
  T update(int a, int b, E x) { return update(a, b, x, 0, 0, n); }
  T query(int a, int b, int k, int l, int r) {
    eval(r - l, k);
    if (r <= a || b <= l) return d1;
    if (a <= l && r <= b) return dat[k];
    T vl = query(a, b, k * 2 + 1, l, (l + r) / 2);
    T vr = query(a, b, k * 2 + 2, (l + r) / 2, r);
    return f(vl, vr);
  }
  T query(int a, int b) { return query(a, b, 0, 0, n); }
};
signed main() {
  int N, M;
  cin >> N >> M;
  vector<tuple<int, int, Int>> Edge;
  for (int i = 0; i < M; i++) {
    int l, r;
    Int c;
    cin >> l >> r >> c;
    l--;
    r--;
    Edge.push_back(make_tuple(l, -r, c));
  }
  sort(Edge.begin(), Edge.end());
  Int INF = 1000000000000000;
  auto f = [](Int a, Int b) { return min(a, b); };
  auto g = [](Int a, Int b) { return a + b; };
  SegmentTree<Int, Int> seg(N, f, f, f, INF, INF, vector<Int>(N, INF),
                            multiplies<Int>());
  seg.update(0, 1, 0);
  for (int i = 0; i < M; i++) {
    int l = get<0>(Edge[i]);
    int r = -get<1>(Edge[i]);
    Int c = get<2>(Edge[i]);
    Int dist = seg.query(l, r + 1);
    seg.update(l, r + 1, dist + c);
  }
  Int ans = seg.query(N - 1, N);
  if (ans == INF) {
    cout << -1 << endl;
  } else {
    cout << ans << endl;
  }
  return 0;
}
