#include <bits/stdc++.h>
#define LLI long long int
#define FOR(v, a, b) for(LLI v = (a); v < (b); ++v)
#define FORE(v, a, b) for(LLI v = (a); v <= (b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define REV(v, a, b) for(LLI v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define ITR(it, c) for(auto it = (c).begin(); it != (c).end(); ++it)
#define RITR(it, c) for(auto it = (c).rbegin(); it != (c).rend(); ++it)
#define EXIST(c,x) ((c).find(x) != (c).end())
#define fst first
#define snd second
#define popcount __builtin_popcount
#define UNIQ(v) (v).erase(unique(ALL(v)), (v).end())
#define bit(i) (1LL<<(i))

#ifdef DEBUG
#include <misc/C++/Debug.cpp>
#else
#define dump(...) ((void)0)
#endif

#define gcd __gcd

using namespace std;
template <class T> constexpr T lcm(T m, T n){return m/gcd(m,n)*n;}

template <typename I> void join(ostream &ost, I s, I t, string d=" "){for(auto i=s; i!=t; ++i){if(i!=s)ost<<d; ost<<*i;}ost<<endl;}
template <typename T> istream& operator>>(istream &is, vector<T> &v){for(auto &a : v) is >> a; return is;}

template <typename T, typename U> bool chmin(T &a, const U &b){return (a>b ? a=b, true : false);}
template <typename T, typename U> bool chmax(T &a, const U &b){return (a<b ? a=b, true : false);}
template <typename T, size_t N, typename U> void fill_array(T (&a)[N], const U &v){fill((U*)a, (U*)(a+N), v);}

struct Init{
  Init(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(12);
    cerr << fixed << setprecision(12);
  }
}init;

template <typename T> class LazySegmentTreeRangeMinimumQuery{
private:
  int size;
  vector<T> vec;
  T e;
  
  inline void propagate(int i){
    if(i<size/2){
      vec[i*2+1] = min(vec[i], vec[i*2+1]);
      vec[i*2+2] = min(vec[i], vec[i*2+2]);
      vec[i] = e;
    }
  }
  
  inline void update_aux(int s, int t, int i, int l, int r, const T &x){
    if(r <= s || t <= l) return;
    else if(s <= l && r <= t) vec[i] = min(vec[i], x);
    else{
      propagate(i);
      update_aux(s,t,i*2+1,l,(l+r)/2,x);
      update_aux(s,t,i*2+2,(l+r)/2,r,x);
    }
  }
  
  inline void get_aux(int i){
    if(i>0) get_aux((i-1)/2);
    propagate(i);
  }
  
public:
  LazySegmentTreeRangeMinimumQuery(int n, const T &e): e(e){
    size = 1;
    while(size < n) size *= 2;
    size = size*2-1;
    vec = vector<T>(size, e);
  }

  inline void update(int s, int t, const T &x){
    update_aux(s,t,0,0,size/2+1,x);
  }

  inline T get(int i){
    int j=i+size/2;
    get_aux((j-1)/2);
    return vec[j];
  }
};



const LLI inf = 1LL << 50;

using Tp = tuple<LLI,LLI,LLI>;

int main(){
  int N,M;
  while(cin >> N >> M){
    vector<LLI> L(M), R(M), C(M);
    REP(i,M){
      cin >> L[i] >> R[i] >> C[i];
      L[i] -= 1;
      R[i] -= 1;
    }

    vector<vector<Tp>> add(N), rmv(N);

    REP(i,M){
      add[L[i]].push_back(make_tuple(L[i], R[i], C[i]));
      rmv[R[i]].push_back(make_tuple(L[i], R[i], C[i]));
    }

    LazySegmentTreeRangeMinimumQuery<LLI> seg(N, inf);

    //vector<LLI> dp(N, INT_MAX);

    //dp[0] = 0;

    seg.update(0, 1, 0);

    REP(i,N){
      LLI d = seg.get(i);
      for(auto &t : add[i]){
        LLI l, r, c;
        tie(l, r, c) = t;

        seg.update(l+1, r+1, d+c);
      }
    }

    LLI ans = seg.get(N-1);

    if(ans == inf){
      cout << -1 << endl;
    }else{
      cout << ans << endl;
    }
  }

  return 0;
}


Measured memory_bytes: 15360