#include <bits/stdc++.h>
using ll = long long;
using namespace std;
int main() {
  ll inf = 1e18;
  int n, m;
  cin >> n >> m;
  ll d[n];
  d[0] = 0;
  for (int i = 1; i < n; ++i) d[i] = inf;
  vector<vector<int> > a(m, vector<int>(3));
  for (int i = 0; i < m; ++i) {
    int l, r, c;
    cin >> l >> r >> c;
    a[i][0] = l - 1;
    a[i][1] = r - 1;
    a[i][2] = c;
  }
  sort(a.begin(), a.end());
  for (int i = 0; i < m; ++i) {
    int l = a[i][0], r = a[i][1], c = a[i][2];
    for (int j = l; j < r; ++j) {
      ll t = d[j] + c;
      if (t < d[r]) d[r] = t;
    }
  }
  cout << (d[n - 1] == inf ? -1 : d[n - 1]) << '\n';
}
