#include <bits/stdc++.h>
using namespace std;
#define rep(i,N) for(int i=0,i##_max=(N);i<i##_max;++i)
#define repp(i,l,r) for(int i=(l),i##_max=(r);i<i##_max;++i)
#define per(i,N) for(int i=(N)-1;i>=0;--i)
#define perr(i,l,r) for(int i=r-1,i##_min(l);i>=i##_min;--i)
#define all(arr) (arr).begin(), (arr).end()
#define SP << " " <<
#define SPF << " "
#define SPEEDUP cin.tie(0);ios::sync_with_stdio(false);
#define MAX_I INT_MAX //1e9
#define MIN_I INT_MIN //-1e9
#define MAX_UI UINT_MAX //1e9
#define MAX_LL LLONG_MAX //1e18
#define MIN_LL LLONG_MIN //-1e18
#define MAX_ULL ULLONG_MAX //1e19
  typedef long long ll;
  typedef pair<int,int> PII;
  typedef pair<char,char> PCC;
  typedef pair<ll,ll> PLL;
  typedef pair<char,int> PCI;
  typedef pair<int,char> PIC;
  typedef pair<ll,int> PLI;
  typedef pair<int,ll> PIL; 
  typedef pair<ll,char> PLC; 
  typedef pair<char,ll> PCL; 

inline void YesNo(bool b){ cout << (b?"Yes" : "No") << endl;}
inline void YESNO(bool b){ cout << (b?"YES" : "NO") << endl;}
inline void Yay(bool b){ cout << (b?"Yay!" : ":(") << endl;}

const ll INF = 1e16;

struct edge{
  int f,t,c;
};

bool comp(const edge& l, const edge& r){
  if(l.t == r.t){
    if(l.c == r.c) return l.f < r.f;
    return l.c < r.c;
  }
  return l.t > r.t;
}

int main(void){
  SPEEDUP
  cout << setprecision(15);
  int N,M;cin >> N >> M;
  vector<edge> v(M);
  vector<edge> w(M);
  rep(i,M) cin >> v[i].f >> v[i].t >> v[i].c;
  sort(all(v),comp);
  vector<ll> cost(N+1,INF);
  cost[N] = 0;
  int index = 0;
  perr(x,1,N+1){
    if(cost[x] == INF)continue;
    while(index<N && v[index].t >= x){
      if(v[index].f > x) continue;
      cost[v[index].f] = min(cost[v[index].f],cost[x]+v[index].c);
      ++index;
    }
  }
  if(v[1] == INF) cout << -1 << endl;
  else cout << cost[1] << endl;
  return 0;
}
