#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <functional>
#include <set>
#include <ctime>
#include <random>
#include <chrono>
#include <cassert>
#include <tuple>
#include <utility>
using namespace std;

namespace {
  using Integer = long long; //__int128;
  template<class T, class S> istream& operator >> (istream& is, pair<T,S>& p){return is >> p.first >> p.second;}
  template<class T> istream& operator >> (istream& is, vector<T>& vec){for(T& val: vec) is >> val; return is;}
  template<class T> istream& operator ,  (istream& is, T& val){ return is >> val;}
  template<class T, class S> ostream& operator << (ostream& os, const pair<T,S>& p){return os << p.first << " " << p.second;}
  template<class T> ostream& operator << (ostream& os, const vector<T>& vec){for(size_t i=0; i<vec.size(); i++) os << vec[i] << (i==vec.size()-1?"":" "); return os;}
  template<class T> ostream& operator ,  (ostream& os, const T& val){ return os << " " << val;}

  template<class H> void print(const H& head){ cout << head; }
  template<class H, class ... T> void print(const H& head, const T& ... tail){ cout << head << " "; print(tail...); }
  template<class ... T> void println(const T& ... values){ print(values...); cout << endl; }

  template<class H> void eprint(const H& head){ cerr << head; }
  template<class H, class ... T> void eprint(const H& head, const T& ... tail){ cerr << head << " "; eprint(tail...); }
  template<class ... T> void eprintln(const T& ... values){ eprint(values...); cerr << endl; }

  class range{ Integer start_, end_, step_; public: struct range_iterator{ Integer val, step_; range_iterator(Integer v, Integer step) : val(v), step_(step) {} Integer operator * (){return val;} void operator ++ (){val += step_;} bool operator != (range_iterator& x){return step_ > 0 ? val < x.val : val > x.val;} }; range(Integer len) : start_(0), end_(len), step_(1) {} range(Integer start, Integer end) : start_(start), end_(end), step_(1) {} range(Integer start, Integer end, Integer step) : start_(start), end_(end), step_(step) {} range_iterator begin(){ return range_iterator(start_, step_); } range_iterator   end(){ return range_iterator(  end_, step_); } };

  inline string operator "" _s (const char* str, size_t size){ return move(string(str)); }
  constexpr Integer my_pow(Integer x, Integer k, Integer z=1){return k==0 ? z : k==1 ? z*x : (k&1) ? my_pow(x*x,k>>1,z*x) : my_pow(x*x,k>>1,z);}
  constexpr Integer my_pow_mod(Integer x, Integer k, Integer M, Integer z=1){return k==0 ? z%M : k==1 ? z*x%M : (k&1) ? my_pow_mod(x*x%M,k>>1,M,z*x%M) : my_pow_mod(x*x%M,k>>1,M,z);}
  constexpr unsigned long long operator "" _ten (unsigned long long value){ return my_pow(10,value); }

  inline int k_bit(Integer x, int k){return (x>>k)&1;} //0-indexed

  mt19937 mt(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count());

  template<class T> string join(const vector<T>& v, const string& sep){ stringstream ss; for(size_t i=0; i<v.size(); i++){ if(i>0) ss << sep; ss << v[i]; } return ss.str(); }

  inline string operator * (string s, int k){ string ret; while(k){ if(k&1) ret += s; s += s; k >>= 1; } return ret; }
}
constexpr long long mod = 9_ten + 7;


typedef struct{
  long long to;
  long long cost;
}edge;
 
 //distance from s
//O(E log V)
template<class T = long long>
void dijkstra(vector<vector<edge> > &G, vector<T> &dist, long long s, const T INF = 1ll<<58){
  
  //first : distance from s, second : its vertex
  priority_queue< pair<T, long long> , vector<pair<T, long long> >, greater< pair<T, long long> > > pq;
   
  fill(dist.begin(), dist.end(), INF);
  dist[s] = 0;
  pq.push( {dist[s], s} );

  while(!pq.empty()){
    auto q = pq.top();
    pq.pop();
    long long from = q.second;
    if(dist[from] < q.first) continue; // it's not minimum distance

    for(auto& e: G[from]){
      if(dist[e.to] > dist[from] + e.cost){
        dist[e.to] = dist[from] + e.cost;
        pq.push( make_pair(dist[e.to], e.to) );
      }
    }
  }
}
 
void add_edge(vector<vector<edge> > &G, long long from, long long to, long long cost){
  G[from].push_back( (edge){to, cost} );
  //G[to].push_back( (edge){from, cost} );
}
 

int main(){
  int n, m;
  cin >> n, m;

  vector<int> l(m), r(m);
  vector<long long> c(m);
  for(int i=0; i<m; i++){
    cin >> l[i],r[i],c[i];
    l[i]--; r[i]--;
  }

  vector<vector<edge>> e(n);
  for(int i=1; i<n; i++){
    add_edge(e, i, i-1, 0);
  }
  for(int i=0; i<m; i++){
    add_edge(e, l[i], r[i], c[i]);
  }

  vector<long long> dist(n);
  dijkstra(e, dist, 0);

  if(dist[n-1] == (1ll<<58)){
    println(-1);
  }else{
    println(dist[n-1]);
  }



  return 0;
}


Measured memory_bytes: 11512