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

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
const ull mod = 1e9 + 7;
#define REP(i,n) for(int i=0;i<(int)n;++i)

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

template<class S, class T> ostream& operator << (ostream& os, const pair<S, T> v){
  os << "(" << v.first << ", " << v.second << ")"; return os;
}
template<class T> ostream& operator << (ostream& os, const vector<T> v){
  for(int i = 0; i < (int)v.size(); i++){if(i > 0){os << " ";} os << v[i];} return os;
}
template<class T> ostream& operator << (ostream& os, const vector<vector<T>> v){
  for(int i = 0; i < (int)v.size(); i++){if(i > 0){os << endl;} os << v[i];} return os;
}

template< typename Monoid >
struct SegmentTree {
  using F = function< Monoid(Monoid, Monoid) >;

  int sz;
  vector< Monoid > seg;

  const F f;
  const Monoid M1;

  // n: 要素数，F: モノイド間の演算，M1: 演算の単位元
  // F は無名関数で渡す
  SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1) {
    sz = 1;
    while(sz < n) sz <<= 1;
    seg.assign(2 * sz, M1);
  }

  // k番目の要素にxを代入
  void set(int k, const Monoid &x) {
    seg[k + sz] = x;
  }

  // 構築
  void build() {
    for(int k = sz - 1; k > 0; k--) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }

  // k番目の値をxに変更
  void update(int k, const Monoid &x) {
    k += sz;
    seg[k] = x;
    while(k >>= 1) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }

  // 区間[a, b)に関して演算して結果を返す
  Monoid query(int a, int b) {
    Monoid L = M1, R = M1;
    for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if(a & 1) L = f(L, seg[a++]);
      if(b & 1) R = f(seg[--b], R);
    }
    return f(L, R);
  }

  // k番目の値を返す(set[k]でアクセス)
  Monoid operator[](const int &k) const {
    return seg[k + sz];
  }


  template< typename C >
  int find_subtree(int a, const C &check, Monoid &M, bool type) {
    while(a < sz) {
      Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
      if(check(nxt)) a = 2 * a + type;
      else M = nxt, a = 2 * a + 1 - type;
    }
    return a - sz;
  }


  template< typename C >
  //[a, x)がcheckを満たす最初の要素位置xを返す
  int find_first(int a, const C &check) {
    Monoid L = M1;
    if(a <= 0) {
      if(check(f(L, seg[1]))) return find_subtree(1, check, L, false);
      return -1;
    }
    int b = sz;
    for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if(a & 1) {
        Monoid nxt = f(L, seg[a]);
        if(check(nxt)) return find_subtree(a, check, L, false);
        L = nxt;
        ++a;
      }
    }
    return -1;
  }

  //[x, b)がcheckを満たす最後の要素位置xを返す
  template< typename C >
  int find_last(int b, const C &check) {
    Monoid R = M1;
    if(b >= sz) {
      if(check(f(seg[1], R))) return find_subtree(1, check, R, true);
      return -1;
    }
    int a = sz;
    for(b += sz; a < b; a >>= 1, b >>= 1) {
      if(b & 1) {
        Monoid nxt = f(seg[--b], R);
        if(check(nxt)) return find_subtree(b, check, R, true);
        R = nxt;
      }
    }
    return -1;
  }
};

struct kukan{
	ll L, R, C;
	bool operator<(const kukan &right)const{
		if(R!=right.R) return (R < right.R);
		else if(L!=right.L) return (L < right.L);
		else return (C < right.C);
	}
};

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll N, M;
    cin >> N >> M;
    vector<kukan> K(M);
    REP(i, M){
    	cin >> K[i].L >> K[i].R >> K[i].C;
    	K[i].L--;
    	K[i].R--;
    }
    sort(K.begin(), K.end());

    SegmentTree<ll> seg(N, [](ll a, ll b){return min(a, b);}, ((1ll<<60)));
    seg.update(0, 0);
    REP(i, M){
    	ll mi = seg.query(K[i].L, K[i].R);
    	ll val = seg[K[i].R];
    	seg.update(K[i].R, min(val, mi+K[i].C));
    }
    ll res = seg[N-1];
    cout << (res<(1ll<<60) ? res : -1) << endl;
    return 0;
}

Measured memory_bytes: 4608