#include <bits/stdc++.h>

using namespace std;

typedef long long LL;
typedef pair<LL, LL> PII;

struct edge {
  LL to, cost;
};

const long long INF = 1LL << 60;
const int MAX_V = 2100000;

int V = MAX_V;
vector<edge> G[MAX_V];
LL d[MAX_V];

void dijkstra(int s) {
  priority_queue<PII, vector<PII>, greater<PII>> que;

  fill(d, d + V, INF);
  d[s] = 0;

  que.push(PII(0, s));

  while (!que.empty()) {
    PII p = que.top();
    que.pop();
    int v = p.second;
    if (d[v] < p.first) continue;

    for (int i = 0; i < G[v].size(); i++) {
      edge e = G[v][i];
      if (d[e.to] > d[v] + e.cost) {
        d[e.to] = d[v] + e.cost;
        que.push(PII(d[e.to], e.to));
      }
    }
  }
}

void add_edge(int from, int to, LL cost) {
  G[from].push_back((edge){to, cost});
}
int main() {
  int N, M;
  cin >> N >> M;

  for (int i = 0; i < N - 1; i++) add_edge(i + 1, i, 0);
  for (int i = 0; i < M; i++) {
    LL l, r, c;
    cin >> l >> r >> c;
    l--;
    r--;

    add_edge(l, r, c);
  }

  dijkstra(0);
  LL ans = d[N - 1];
  if (ans ==  INF) ans = -1;
  cout << ans << endl;
  return 0;
}