#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<string>
#include<string.h>
#include<math.h>
#include<stdlib.h>
#include<vector>
#include<queue>
#include<map>
#include<iomanip>
#include<set>
#include<bitset>
#include "bits/stdc++.h"
#define rep(index,num) for(int index=0;index<num;index++)
#define rep1(index,num) for(int index=1;index<=num;index++)
#define brep(index,num) for(int index=num-1;index>=0;index--)
#define brep1(index,num) for(int index=num;index>0;index--)
#define scan(argument) cin>>argument
#define prin(argument) cout<<argument<<endl
#define kaigyo cout<<endl
#define eps 1e-7
#define mp(a1,a2) make_pair(a1,a2)
#define ALL(a) (a).begin(),(a).end()
#define rALL(a) (a).rbegin(),(a).rend()
typedef long long ll;
typedef long double ld;
using namespace std;
typedef pair<ll,ll> pll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<pint> vpint;
typedef vector<pll> vpll;
ll INFl=1e+18+1;
int INF=1e+9+1;/*
struct LazySegmentTree {
private:
    ll n;
    vector<ll> node, lazy;
    vector<bool> lazyFlag;

public:
    LazySegmentTree(vector<ll> v) {
        int sz = (int)v.size();
        n = 1; while(n < sz) n *= 2;
        node.resize(2*n-1);
        lazy.resize(2*n-1, INFl);
        lazyFlag.resize(2*n-1, false);

        for(int i=0; i<sz; i++) node[i+n-1] = v[i];
        for(int i=n-2; i>=0; i--) node[i] = min(node[i*2+1], node[i*2+2]);
    }

    void lazyEvaluate(ll k, ll l, ll r) {
        if(lazyFlag[k]) {
            node[k] = lazy[k];
            if(r - l > 1) {
                lazy[k*2+1] = lazy[k*2+2] = lazy[k];
                lazyFlag[k*2+1] = lazyFlag[k*2+2] = true;
            }
            lazyFlag[k] = false;
        }
    }

    void update(ll a, ll b, ll x, ll k=0, ll l=0, ll r=-1) {
        if(r < 0) r = n;
        lazyEvaluate(k, l, r);
        if(b <= l || r <= a) return;
        if(a <= l && r <= b) {
            lazy[k] = x;
            lazyFlag[k] = true;
            lazyEvaluate(k, l, r);
        }
        else {
            update(a, b, x, 2*k+1, l, (l+r)/2);
            update(a, b, x, 2*k+2, (l+r)/2, r);
            node[k] = min(node[2*k+1], node[2*k+2]);
        }
    }

    ll find(ll a, ll b, ll k=0, ll l=0, ll r=-1) {
        if(r < 0) r = n;
        lazyEvaluate(k, l, r);
        if(b <= l || r <= a) return INFl;
        if(a <= l && r <= b) return node[k];
        ll vl = find(a, b, 2*k+1, l, (l+r)/2);
        ll vr = find(a, b, 2*k+2, (l+r)/2, r);
        return min(vl, vr);
    }
};*/
template< typename Monoid >
struct SegmentTree {
  using F = function< Monoid(Monoid, Monoid) >;

  ll sz;
  vector< Monoid > seg;

  const F f;
  const Monoid M1;

  SegmentTree(ll n, const F f, const Monoid &M1) : f(f), M1(M1) {
    sz = 1;
    while(sz < n) sz <<= 1;
    seg.assign(2 * sz, M1);
  }

  void set(ll k, const Monoid &x) {
    seg[k + sz] = x;
  }

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

  void update(ll k, const Monoid &x) {
    k += sz;
    seg[k] = x;
    while(k >>= 1) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }

  void add(ll k, const Monoid &x) {
	k += sz;
	seg[k] += x;
	while(k >>= 1) {
	  seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
	}
  }

  Monoid query(ll a, ll b) {//半開区間
    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);
  }

  Monoid operator[](const ll &k) const {
    return seg[k + sz];
  }
};
int main(){
	int N,M,L[100001],R[100001];
	ll C[100001];
	vector<pair<pll,ll> > LRC;
	scan(N>>M);
	//LazySegmentTree seg( vector<ll>(N, INFl) );
	SegmentTree<ll> seg(N,[](ll a, ll b){return min(a,b);},INFl);
	seg.update(0,0);
	rep(i,M){
		scan(L[i]>>R[i]>>C[i]);
		L[i]--;
		R[i]--;
		LRC.push_back({{L[i],R[i]},C[i]});
	}
	sort(ALL(LRC));
	rep(i,M){
		ll l=LRC[i].first.first;
		ll r=LRC[i].first.second;
		ll c=LRC[i].second;
		ll hidari=seg.query(l,N);
		ll x=min(hidari+c,seg.query(r,r+1));
		seg.update(r,x);
	}
	ll ans=seg.query(N-1,N);
	if(ans==INFl) prin(-1);
	else prin(ans);
	return 0;
}


Measured memory_bytes: 7924