#include <bits/stdc++.h>
using namespace std;
const long long oo = 1LL << 60;

int n;
vector<pair<int, int>> a[100100];
vector<pair<long long, int>> out[100100];

bool cmp(pair<int, int> u, pair<int, int> v)
{
  if (u.first != v.first)
    return u.first < v.first;
  return u.second > v.second;
}

int main()
{
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int m;
  cin >> n >> m;
  while (m--)
  {
    int l, r, c;
    cin >> l >> r >> c;
    a[l].push_back({c, r});
  }

  set<pair<long long, int>> s;
  for (int i = 1; i <= n; i++)
  {
    long long dist = oo;
    if (i == 1) dist = 0;
    else if (s.empty()) dist = oo;
    else dist = s.begin()->first;

    if (i == n)
    {
      cout << (dist == oo ? -1 : dist) << endl;
      return 0;
    }

    if (dist != oo)
    {
      sort(a[i].begin(), a[i].end(), cmp);
      int r = i;
      for (auto u : a[i])
        if (u.second > r)
        {
          s.insert({dist + u.first, i});
          out[u.second].push_back({dist + u.first, i});
          r = u.second;
        }
    }

    for (auto u : out[i])
      s.erase(u);
  }
}
