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

using i64 = int64_t;

constexpr i64 INF = 1e17;

struct Node{
    i64 l, r, c;
    bool operator<(const Node &obj)const{
        if(l == obj.l)return r < obj.r;
        return l < obj.l;
    }
};

struct State{
    i64 num, cost;
    bool operator<(const State &obj)const{
        return cost > obj.cost;
    }
};

int main(){
    int n, m;
    cin >> n >> m;
    vector<Node> v;
    v.push_back({0, 1, 0});
    for(int i=0;i<m;++i){
        i64 l, r, c;
        cin >> l >> r >> c;
        v.push_back({l, r, c});
    }
    sort(v.begin(), v.end());
    vector<i64> mini(m+1, INF);
    mini[0] = 0;
    priority_queue<State> que;
    que.push({0, 0});
    while(!que.empty()){
        auto cur = que.top(); que.pop();
        if(v[cur.num].r < rmax)continue;
        rmax = cur.num;
        //cout << cur.num << " " << cur.cost << endl;
        if(v[cur.num].r == n){
            cout << cur.cost << endl;
            return 0;
        }
        for(int i=cur.num+1;i<=m;++i){
            if(v[i].l > v[cur.num].r)break;
            if(v[i].l == v[cur.num].l)continue;
            i64 new_cost = cur.cost + v[i].c;
            if(new_cost >= mini[i])continue;
            mini[i] = new_cost;
            que.push({i, new_cost});
        }
    }
    cout << -1 << endl;

    return 0;
}
