#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <array>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <stack>
#include <queue>
#include <bitset>
#include <tuple>
#include <cmath>
#include <complex>
#include <algorithm>
#include <utility>
#include <regex>
#include <cstdint>
#include <numeric>
#include <functional>
#include <cassert>

using namespace std;
using ll = long long;
using ull = unsigned long long;

constexpr ll INF = 1'000'000'000'000'000'000;
namespace utils{
#define ALL(x) begin(x), end(x)
#define RALL(x) rbegin(x), rend(x)
    template<class T, class Compare> using p_queue = priority_queue<T, vector<T>, Compare>;
    template<class T> using min_queue = p_queue<T, greater<T>>;
    template<class T> using max_queue = p_queue<T, less<T>>;

    template<class T> bool up_min(T& X, const T& A){ if(X > A) {X = A; return true;} return false; }
    template<class T> bool up_max(T& X, const T& A){ if(X < A) {X = A; return true;} return false; }

    ll operator"" _64(unsigned long long x){ return ll(x); }
    ull operator"" _64u(unsigned long long x){ return ull(x); }

    template<class T>
    vector<T> make_mdvector(int n, T t){
        return vector<T>(n, t);
    }
    template<class ...Ts>
    auto make_mdvector(int n, Ts ... ts){
        return vector<decltype(make_mdvector(ts...))>(n, make_mdvector(ts...));
    }

    inline ll bit(int x){
        return 1ll << x;
    }

    template<class T>
    istream& operator>>(istream& is, vector<T>& v){
        for (auto &&x : v) { is >> x; } return is;
    }

    template<class T>
    ostream& operator<<(ostream& os, const vector<T>& v){
        for (auto &&x : v) { os << x << ' '; } cout << endl; return os ;
    }
}
using namespace utils;

class solver{
    istream& is;
    ostream& os;

public:
    solver(istream& I, ostream& O) :is(I), os(O) {}
    ll N;
    void run();
};

int main(int argc, char *argv[]) {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << setprecision(16) << scientific;
#ifdef XELMH
    string test_cases = "test_C.txt";
    cerr << test_cases << " -->" << endl;
    auto fs = fstream(test_cases, fstream::in);
    int loop = 0;
    while(fs) {
        loop++;
        cout << '#' << loop << "#------\n";
        solver(fs, cout).run();
    }
    if(loop <= 1) {
        cout << "===" << endl;
        while(cin) solver(cin, cout).run();
    }
#else
    solver(cin, cout).run();
#endif
    return 0;
}


void solver::run(){
    ll M;
    is >> N >> M;
    if(!is) return;
    vector<tuple<ll, ll, ll>> lcb(M);
    for (int i = 0; i < M; ++i) {
        ll a, b, c;
        is >> a >> b >> c;
        lcb[i] = tie(a, c, b);
    } // end i
    sort(ALL(lcb));
    min_queue<tuple<ll, ll>> drq;
    drq.emplace(0, 1);
    for (auto &&lll : lcb) {
        ll li, ci, ri;
        tie(li, ci, ri) = lll;
        if(ri < get<1>(drq.top())) continue;
        while(li > get<1>(drq.top())){
            drq.pop();
            if(drq.empty()){
                os << -1 << endl;
                return;
            }
        }
        ll d = get<0>(drq.top()) + ci;
        drq.emplace(d, ri);
    } // end lll
    ll d, p;
    if(drq.empty()){
        os << -1 << endl;
        return;
    }
    tie(d, p) = drq.top();
    while(p < N){
        drq.pop();
        if(drq.empty()){
            os << -1 << endl;
            return;
        }
        tie(d, p) = drq.top();
    }
    os << d << endl;

}


Measured memory_bytes: 4596