#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int INF = 1e9;
const ll LINF = 1e18;
inline ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
inline ll lcm(ll a, ll b) { return a / gcd(a, b)*b; }
template<class S,class T> ostream& operator << (ostream& out,const pair<S,T>& o){ out << "(" << o.first << "," << o.second << ")"; return out; }
template<class T> ostream& operator << (ostream& out,const vector<T>& V){ for(int i = 0; i < V.size(); i++){ out << V[i]; if(i!=V.size()-1) out << " ";} return out; }
template<class T> ostream& operator << (ostream& out,const vector<vector<T> >& Mat){ for(int i = 0; i < Mat.size(); i++) { if(i != 0) out << endl; out << Mat[i];} return out; }
template<class S,class T> ostream& operator << (ostream& out,const map<S,T>& mp){ out << "{ "; for(auto it = mp.begin(); it != mp.end(); it++){ out << it->first << ":" << it->second; if(mp.size()-1 != distance(mp.begin(),it)) out << ", "; } out << " }"; return out; }
template<typename T>vector<T> make_v(size_t a){return vector<T>(a);}
template<typename T,typename... Ts>auto make_v(size_t a,Ts... ts){return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));}
template<typename T,typename V> typename enable_if<is_class<T>::value==0>::type fill_v(T &t,const V &v){t=v;}
template<typename T,typename V> typename enable_if<is_class<T>::value!=0>::type fill_v(T &t,const V &v){for(auto &e:t) fill_v(e,v);}
/*
 <url:https://atcoder.jp/contests/nikkei2019-2-qual/tasks/nikkei2019_2_qual_d>
 問題文============================================================
 =================================================================
 解説=============================================================
 ================================================================
 */

template<class Type>
Type solve(Type res = Type()){
    int N,M; cin >> N >> M;
    vector<vector<pll>> G(N);
    vector<int> imos(N+1);
    for(int i = 0; i < M;i++){
        int L,R,C; cin >> L >> R >> C;
        L--; R--;
        G[L].push_back(pll(R,C));
        
        imos[L]++;
        imos[R]--;
    }
    for(int i = 0; i < N;i++){
        imos[i+1] += imos[i];
    }
    
    for(int i = 0; i < N;i++){
        if(imos[i]) G[i+1].push_back(pll(i,0));
    }
    vector<ll> dist(N,LINF);
    queue<ll> q;
    q.push(0);
    dist[0] = 0;
    while(q.size()){
        ll n = q.front(); q.pop();
        for(auto p:G[n]){
            if(dist[p.first] > dist[n] + p.second){
                dist[p.first] = dist[n] + p.second;
                q.push(p.first);
            }
        }
    }
    res = dist[N-1];
    if(res == LINF) res = -1;
    return res;
}
int main(void) {
    cin.tie(0); ios_base::sync_with_stdio(false);
    // solve(0);
    cout << fixed << setprecision(15) << solve<ll>() << endl;
    return 0;
}
