#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  vector<vector<long long>> in(n, vector<long long>(0)),
      out(n, vector<long long>(0));
  vector<long long> c(m);
  for (int i = 0; i < m; i++) {
    int l, r;
    cin >> l >> r >> c[i];
    in[l - 1].push_back(i);
    out[r - 1].push_back(i);
  }
  vector<long long> dist(n, 1LL << 48), dist_c(m, 1LL << 48);
  dist[0] = 0;
  set<long long> dist_set;
  for (int i = 0; i < n; i++) {
    if (dist_set.size() > 0) {
      dist[i] = min(dist[i], *dist_set.begin());
    }
    for (int j = 0; j < static_cast<int>(in[i].size()); j++) {
      dist_c[in[i][j]] = dist[i] + c[in[i][j]];
      dist_set.insert(dist_c[in[i][j]]);
    }
    for (int j = 0; j < static_cast<int>(out[i].size()); j++) {
      dist_set.erase(dist_set.find(dist_c[out[i][j]]));
    }
  }
  if (dist[n - 1] < (1LL << 48)) {
    cout << dist[n - 1] << endl;
  } else {
    cout << "-1" << endl;
  }
}
