#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

int main() {
  long long inf = 1e18;
  int n, m;
  cin >> n >> m;
  long long 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 (const auto& e : a) {
    int l = e[0], r = e[1], c = e[2];
    for (int j = l; j < r; ++j) {
      long long t = d[j] + c;
      if (t < d[r]) d[r] = t;
    }
  }
  if (d[n-1] == inf) cout << -1 << endl;
  else cout << d[n-1] << endl;
}