#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
ll const INF = ll(1e18);
struct Seq {
  int l;
  int r;
  ll c;
};
bool operator<(Seq const& lhs, Seq const& rhs) {
  if (lhs.l == rhs.l) {
    if (lhs.r == rhs.r) {
      return lhs.c < rhs.c;
    }
    return lhs.r < rhs.r;
  }
  return lhs.l < rhs.l;
}
struct RNGT {
  int nd;
  vector<ll> d;
  RNGT(int n) {
    nd = n;
    d.resize(nd * 4, INF);
  }
  void update(int qp, ll val) { return __update(1, 0, nd - 1, qp, val); }
  void __update(int p, int l, int r, int qp, ll val) {
    if (qp < l || r < qp) return;
    if (l == r) {
      d[p] = min(d[p], val);
      return;
    }
    int mid = l + (r - l) / 2;
    __update(p * 2, l, mid, qp, val);
    __update(p * 2 + 1, mid + 1, r, qp, val);
    d[p] = min(d[p * 2], d[p * 2 + 1]);
  }
  ll query(int ql, int qr) { return __query(1, 0, nd - 1, ql, qr); }
  ll __query(int p, int l, int r, int ql, int qr) {
    if (r < ql || qr < l) return INF;
    if (l == r) return d[p];
    int mid = l + (r - l) / 2;
    return min(__query(p * 2, l, mid, ql, qr),
               __query(p * 2 + 1, mid + 1, r, ql, qr));
  }
};
int main() {
  int n, m;
  cin >> n >> m;
  vector<Seq> vs;
  int l, r, c;
  for (int i = 0; i < m; ++i) {
    cin >> l >> r >> c;
    --l, --r;
    vs.push_back({l, r, c});
  }
  sort(vs.begin(), vs.end());
  RNGT segt(n);
  segt.update(0, 0);
  for (int i = 0; i < m; ++i) {
    segt.update(vs[i].r, segt.query(vs[i].l, vs[i].r - 1) + vs[i].c);
  }
  ll ans = segt.query(n - 1, n - 1);
  cout << (ans == INF ? -1 : ans) << endl;
}
