#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <numeric>

#define REP(i, a, b) for (int i = int(a); i < int(b); i++)
using namespace std;
typedef long long int ll;

// clang-format off
#ifdef _DEBUG_
#define dump(...) do{ cerr << __LINE__ << ":\t" << #__VA_ARGS__ << " = "; PPPPP(__VA_ARGS__); cerr << endl; } while(false)
template<typename T> void PPPPP(T t) { cerr << t; }
template<typename T, typename... S> void PPPPP(T t, S... s) { cerr << t << ", "; PPPPP(s...); }
#else
#define dump(...)
#endif
template<typename T> vector<T> make_v(size_t a, T b) { return vector<T>(a, b); }
template<typename... Ts> auto make_v(size_t a, Ts... ts) { return vector<decltype(make_v(ts...))>(a, make_v(ts...)); }
template<typename T>
bool chmin(T &a, T b) { if (a > b) {a = b; return true; } return false;}
template<typename T>
bool chmax(T &a, T b) { if (a < b) {a = b; return true; } return false;}
// clang-format on

#include <tuple>
using T = tuple<int, int, int>;

#include <functional>
template<typename T>
class SegmentTreeOneAll {
    using Func = function<T(T, T)>;

public:
    vector<T> data;
    int n;
    T init;
    Func update_func;
    Func query_func;
    SegmentTreeOneAll(int _n, T _init, Func up, Func qu) {
        init = _init;
        update_func = up;
        query_func = qu;
        for (n = 1; n < _n; n *= 2)
            ;
        data.resize(2 * n - 1, init);
    }
    void update(int pos, T val) {
        pos += n - 1;
        data[pos] = update_func(data[pos], val);
        while (pos > 0) {
            pos = (pos - 1) / 2;
            data[pos] = query_func(data[2 * pos + 1], data[2 * pos + 2]);
        }
    }
    T query(int l, int r) {
        T res = init;
        for (l += n - 1, r += n - 1; l < r; l = l / 2, r = (r - 1) / 2) {
            if (!(l & 1)) {
                res = query_func(res, data[l]);
            }
            if (!(r & 1)) {
                res = query_func(res, data[r - 1]);
            }
        }
        return res;
    }
};

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    vector<T> d;
    REP(i, 0, m) {
        int a, b, c;
        cin >> a >> b >> c;
        d.emplace_back(b, a, c);
    }
    sort(d.begin(), d.end());
    auto MIN = [](ll a, ll b) {
        return min(a, b);
    };
    const ll inf = 1LL << 30;
    SegmentTreeOneAll<ll> segtree(n + 1, inf, MIN, MIN);
    segtree.update(1, 0);
    REP(i, 0, m) {
        int L, R, C;
        tie(R, L, C) = d[i];
        ll mi = segtree.query(L, R + 1);
        segtree.update(R, mi + C);
    }
    ll v = segtree.query(n, n + 1);
    if (v >= inf) {
        cout << -1 << endl;
    } else {
        cout << v << endl;
    }
    return 0;
}
