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

int main(){
    int n, m;
    cin >> n >> m;
    vector<pair<int,int64_t>> edges[100000];
    for(int i=0;i<m;++i){
        int l, r, c;
        cin >> l >> r >> c;
        edges[l-1].emplace_back(r-1, c);
    }
    for(int i=1;i<n;++i) edges[i].emplace_back(i-1, 0);

    priority_queue<pair<int64_t,int>> q;
    q.emplace(0, 0);
    bool checked[100000] = {0};
    while(!q.empty()){
        auto t = q.top();
        q.pop();
        if(checked[t.second]) continue;
        checked[t.second] = true;
        if(t.second == n-1){
            cout << (-t.first) << endl;
            return 0;
        }
        for(auto to : edges[t.second]){
            if(checked[to.first]) continue;
            q.emplace(t.first-to.second, to.first);
        }
    }
    cout << -1 << endl;

    return 0;
}

/*

*/
