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*/

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

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a)  begin((a)),end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define FF first
#define SS second

template<class S, class T>
istream& operator>>(istream& is, pair<S,T>& p){
  return is >> p.FF >> p.SS;
}
template<class T>
istream& operator>>(istream& is, vector<T>& xs){
  for(auto& x: xs)
	is >> x;
  return is;
}
template<class S, class T>
ostream& operator<<(ostream& os, const pair<S,T>& p){
  return os << p.FF << " " << p.SS;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T>& xs){
  for(unsigned int i=0;i<xs.size();++i)
	os << (i?" ":"") << xs[i];
  return os;
}
template<class T>
void maxi(T& x, T y){
  if(x < y) x = y;
}
template<class T>
void mini(T& x, T y){
  if(x > y) x = y;
}
void debug(istringstream&){}
template <char sep=',', class Head, class... Tail>
void debug(istringstream& iss, Head&& head, Tail&&... tail)
{
  string name;
  getline(iss, name, ',');
  cout << sep << name << "=" << head;
  debug(iss, forward<Tail>(tail)...);
}

#ifdef PYONPOI
#define DEBUG(...) \
  do{ \
    istringstream ss(#__VA_ARGS__); \
    debug<' '>(ss, __VA_ARGS__); \
    cout<<endl; \
  }while(0)
#else
#define DEBUG
#endif
void init_io(int argc, char* argv[]){
  cin.tie(0);
  ios_base::sync_with_stdio(false);
#ifdef PYONPOI
  if(argc > 1) {
    ifstream *ifs = new ifstream();
    ifs->open(argv[1]);
    cin.rdbuf(ifs->rdbuf());
  }
#endif
}


const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1e9+7;
const LL INF = 1e16;

template<class T>
class LazySegT{
private:
  int NN;
  struct Node{
    T dat;
    T lazy;
  };
  Node *segT;
public:
  LazySegT(int n){
    NN = 1;
    while(NN < n) NN <<= 1;
    segT = new Node[2 * NN];

    for(int i=0;i<2*NN-1;++i)
      segT[i].dat = INF, segT[i].lazy = INF;
  }

  ~LazySegT(){
    delete segT;
  }

  void eval(int k, int l, int r){
    if(segT[k].lazy == INF) return;
    
    mini(segT[k].dat, segT[k].lazy);
    if(k < NN-1){ // not leaf
      mini(segT[2*k+1].lazy, segT[k].lazy);
      mini(segT[2*k+2].lazy, segT[k].lazy);
    }
    segT[k].lazy = INF;
  }

  void update(int a, int b, T c, int k, int l, int r){
    eval(k,l,r);
    if(r <= a || b <= l) return;

    if(a <= l && r <= b){
      mini(segT[k].lazy, c);
      eval(k,l,r);
    }
    else{
      update(a, b, c, k*2+1, l, (l+r)/2);
      update(a, b, c, k*2+2, (l+r)/2, r);
      segT[k].dat = min(segT[k*2+1].dat, segT[k*2+2].dat);
    }
  }
  // dat[idx] = c, a <= idx < b
  void update(int a, int b, T c){
    update(a, b, c, 0, 0, NN);
  }

  T query(int a, int b, int k, int l, int r){
    eval(k,l,r);
    if(r <= a || b <= l) return INF;

    if(a <= l && r <= b) return segT[k].dat;
    else{
      T vl = query(a, b, k*2+1, l, (l+r)/2);
      T vr = query(a, b, k*2+2, (l+r)/2, r);
      segT[k].dat = min(segT[k*2+1].dat, segT[k*2+2].dat);
      return min(vl, vr);
    }
  }
  T query(int a, int b){
    return query(a, b, 0, 0, NN);
  }
};


int main(int argc, char* argv[]){
  init_io(argc, argv);

  int N, M;
  cin >> N >> M;

  LazySegT<LL> seg(N);
  vector<pair<PII,LL>> es(M);

  REP(i,M){
    cin >> es[i].FF.FF >> es[i].FF.SS >> es[i].SS;
    es[i].FF.FF--;
  }
  SORT(es);

  seg.update(0, 1, 0);
  for(auto e : es){
    auto l = e.FF.FF;
    auto r = e.FF.SS;
    auto c = e.SS;
    LL up = seg.query(l, l+1) + c;
    seg.update(l, r, up);
  }

  auto ans = seg.query(N-1, N);
  cout << (ans == INF ? -1 : ans) << endl;

  return 0;
}


Measured memory_bytes: 5888