#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<ll,ll> P;
const ll INF = 1LL << 50;

vector<vector<P>> to; vector<ll> to_dist;

void Dijkstra(ll v){
  
  to_dist[v] = 0; 
  
  priority_queue<P, vector<P>, greater<P>> Q; 
  Q.push({0,v}); 
  
  while(!Q.empty()){
   
    P p = Q.top(); Q.pop();
    ll s = p.first; ll t = p.second; 
      for(auto ne: to[t]){
        ll x = ne.first; ll y = ne.second;
        
        if( to_dist[x] > to_dist[t] + y){
          to_dist[x] = to_dist[t] + y; Q.push({to_dist[x], x});}
      }
  }

}


int main(){
  ll N,M; cin >> N >> M;
  to.resize(N); to_dist = vector<ll> (N,INF);
  
  for(ll i = 0; i < N-1; i++){  to[i+1].push_back({i,0});}
  
  for(ll i = 0; i < M; i++){
    ll L,R,C; cin >> L >> R >> C; L--; R--; 
    to[L].push_back({R,C}); to[R].push_back({L,C});
  }
  
  Dijkstra(0);
  ll ans = to_dist[N-1]; 
  if( ans == INF){ ans = -1;} 
  
  cout << ans << endl; return 0;
}
    
  
 
  