#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR if(DEBUG) cerr
#define MACRO(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,NAME,...) NAME
#define pr(...) CERR<<MACRO(__VA_ARGS__,pr10,pr9,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__)<<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define pr9(a,b,c,d,e,f,g,h,i) pr1(a)<<pr8(b,c,d,e,f,g,h,i)
#define pr10(a,b,c,d,e,f,g,h,i,j) pr1(a)<<pr9(b,c,d,e,f,g,h,i,j)
#define prArr(a){CERR<<(#a)<<"={";int i=0;for(auto t:(a))CERR<<(i++?", ":"")<<t;CERR<<"}"<<endl;}
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE

template<typename dtype, dtype initValue /*範囲外の時に返す値*/>
class RMQ{
public :

  struct data{
    bool type; //0 - empty   , 1 - update
    dtype value;
  };

  Int n,n_;
  vector<dtype> dat;
  vector<data> td;
  Int toMax;          //0 -> RangeMin 1->RangeMax;

  RMQ(){n=-1;}
  RMQ(Int n_, Int toMax = 0):n_(n_),toMax(toMax){
    n=1;
    while(n<n_)n*=2;
    td.resize(2*n-1,(data){0,initValue});
    dat.resize(2*n-1,initValue);
  }

  inline dtype merge(dtype a, dtype b){return toMax? max(a, b):min(a, b);}


  dtype dfs(Int a,Int b,dtype x,bool flag,Int k,Int l,Int r){
    if(r <= a || b <= l) return flag? dat[k]:initValue;
    if(a <= l && r <= b){
      if(flag == true){
        td[k].type = 1;
        Min(td[k].value, x);
        Min(dat[k], x);
      }
      return dat[k];
    }

    if(td[k].type){
      td[k].type = 0;
      Min(dat[k*2+1], td[k].value);
      Min(dat[k*2+2], td[k].value);

      td[k*2+1].type = td[k*2+2].type = 1;
      Min(td[k*2+1].value, td[k].value);
      Min(td[k*2+2].value, td[k].value);
      td[k].value = INF;
    }

    dtype vl=dfs(a, b, x, flag, k*2+1, l, (l+r)/2);
    dtype vr=dfs(a, b, x, flag, k*2+2, (l+r)/2, r);
    return flag? (dat[k] = merge(vl, vr)):merge(vl,vr);
  }

  //[l,r)の値をxに変更　update(l,r,x)
  void update(Int l,Int r,dtype x){
    assert(l <= r), assert(l <= n && r <= n), assert(l >= 0 && r >= 0);
    dfs(l, r, x, true, 0, 0, n);
  }

  //[l,r)の最小値を得る　find(l,r);
  dtype get(Int l,Int r){
    assert(l <= r), assert(l <= n && r <= n), assert(l >= 0 && r >= 0);
    return dfs(l, r, initValue , false, 0 , 0 ,n);
  }
};

signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  Int N, M;
  cin>>N>>M;
  using T = tuple<Int,Int,Int>;
  vector<T> A;
  for(Int i=0;i<M;i++){
    Int l, r, c;
    cin>>l>>r>>c;
    l--, r--;
    A.emplace_back(l, r, c);
  }
  sort(A.begin(), A.end());



  RMQ<Int, INF> seg(N);
  seg.update(0, 1, 0);

  Int idx = 0;
  for(Int i=0;i<N;i++){
    Int dis = seg.get(i, i+1);
    if(dis == INF) continue;
    while(idx < M && get<0>(A[idx]) == i){
      Int l, r, c; tie(l, r, c) = A[idx++];
      Int ndis = dis + c;
      seg.update(l, r+1, ndis);
    }
  }
  Int ans = seg.get(N-1, N);
  if(ans == INF) ans = -1;
  cout<<ans<<endl;


  return 0;
}


Measured memory_bytes: 10736