#include <iostream>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
using pli=pair<long, int>;
using mii=map<int, int>;

struct node {
    mii edge;
    bool done = false;
    int64_t cost = -1;

    bool operator<(const node &rhs) const {
        if (rhs.cost == -1) {
            return true;
        } else if (this->cost == -1) {
            return false;
        }
        return this->cost < rhs.cost;
    }

    bool operator>(const node &rhs) const {
        return rhs < *this;
    }

    void update_cost(int64_t x) noexcept {
        if (x < 0) {
            cerr << "Illegal Argument in node.update_cost: x=" << x << endl;
            exit(1);
        }
        if (this->cost == -1 || this->cost > x) {
            this->cost = x;
        }
    }
};

int main() {
    int N, M;
    cin >> N >> M;

    int low = 0, high = 0;
    vector<node> nodes(N);
    for (auto i = 0; i < M; i++) {
        int L, R, C;
        cin >> L >> R >> C;
        if (nodes.at(L - 1).edge.count(R - 1) == 0) {
            nodes.at(L - 1).edge[R - 1] = C;
        } else {
            nodes.at(L - 1).edge[R - 1] = min(nodes.at(L - 1).edge[R - 1], C);
        }
        if (nodes.at(R - 1).edge.count(L - 1) == 0) {
            nodes.at(R - 1).edge[L - 1] = C;
        } else {
            nodes.at(R - 1).edge[L - 1] = min(nodes.at(R - 1).edge[L - 1], C);
        }

        if (i == 0) {
            low = L - 1;
            high = R - 1;
        }
        if (L - 1 < low && R - 1 >= low) {
            low = L - 1;
        }
        if (R - 1 > high && L - 1 <= high) {
            high = R - 1;
        }
    }

    if (low != 0 || high != N - 1) {
        cout << -1 << endl;
        return 0;
    }

    for (auto i = N - 1; i > 0; i--) {
        nodes.at(i).edge[i - 1] = 0;
    }

    priority_queue<pli, vector<pli>, greater<>> q;

    nodes.at(0).cost = 0;
    q.emplace(0, 0);

    do {
        auto top = q.top();
        q.pop();
        nodes.at(top.second).done = true;
        for (auto e:nodes.at(top.second).edge) {
            nodes.at(e.first).update_cost(top.first + e.second);
            if (!nodes.at(e.first).done && nodes.at(e.first).cost != -1) {
                q.emplace(nodes.at(e.first).cost, e.first);
            }
        }
    } while (!q.empty());

    cout << nodes.at(N - 1).cost << endl;
    return 0;
}
