#include <bits/stdc++.h>
using namespace std;
const long long MAXN = 1e5 + 10, inf = 1e18 + 10;
int n, m;
int l, r, c;
long long d[MAXN];
set<pair<long long, long long> > st;
vector<pair<long long, long long> > adj[MAXN];
void dijk() {
  for (int i = 2; i <= n; i++) d[i] = inf;
  st.insert({0, 1});
  while (st.size()) {
    int x = st.begin()->second;
    st.erase(st.begin());
    for (auto y : adj[x])
      if (d[y.first] > d[x] + y.second) {
        st.erase({d[y.first], y.first});
        d[y.first] = d[x] + y.second;
        st.insert({d[y.first], y.first});
      }
  }
}
int main() {
  for (int i = n; i > 1; i--) adj[i].push_back({i - 1, 0});
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    cin >> l >> r >> c;
    adj[l].push_back({r, c});
  }
  dijk();
  if (d[n] == inf)
    cout << -1 << '\n';
  else
    cout << d[n] << '\n';
  return 0;
}
