#include <bits/stdc++.h>

#include <utility>

using namespace std;

using ll = long long;
using P = pair<ll, ll>;
using Graph = vector<vector<ll>>;
#define rep(i, n) for(ll i=0;i<(ll)(n);i++)
#define rep2(i, m, n) for(ll i=m;i<(ll)(n);i++)
#define rrep(i, n, m) for(ll i=n;i>=(ll)(m);i--)
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const int ddx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const int ddy[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll MOD = 1000000007;
const ll INF = 1000000000000000000L;

#ifdef __DEBUG

/**
 * For DEBUG
 * https://github.com/ta7uw/cpp-pyprint
 */
#include "cpp-pyprint/pyprint.h"

#endif

template<class Monoid>
class SegmentTree {

public:
    /**
     * @param N size
     * @param e identity element
     * @param operation operation to merge `data`
     * @param updater operation to update `data`
     */
    SegmentTree(size_t N, Monoid e, function<Monoid(Monoid, Monoid)> operation,
                function<Monoid(Monoid, Monoid)> updater)
            : e(e), operation(operation), updater(move(updater)) {
        n = 1;
        while (n < N) {
            n *= 2;
        }
        data = vector<Monoid>(2 * n - 1, e);
        for (int i = n - 2; i >= 0; i--) {
            data[i] = operation(data[2 * i + 1], data[2 * i + 2]);
        }
    }

    /**
     * iの値をxに更新 ( 0-indexed )
     */
    void update(int i, Monoid x) {
        i += n - 1;
        data[i] = updater(data[i], x);
        while (i > 0) {
            i = (i - 1) / 2;
            data[i] = operation(data[i * 2 + 1], data[i * 2 + 2]);
        }
    }

    /**
     * [a, b)の区間でクエリを実行
     */
    Monoid query(int a, int b) {
        return query(a, b, 0, 0, n);
    }

    /**
     * 添字でアクセス( 0-indexed )
     */
    Monoid operator[](int i) {
        return data[i + n - 1];
    }

private:
    int n;
    vector<Monoid> data;
    Monoid e;
    function<Monoid(Monoid, Monoid)> operation;
    function<Monoid(Monoid, Monoid)> updater;

    Monoid query(int a, int b, int k, int l, int r) {
        // 交差しない
        if (r <= a || b <= l) return e;
        // 区間 [a, b) に l, r が含まれる
        if (a <= l && r <= b) return data[k];
        // 左の子
        Monoid c1 = query(a, b, 2 * k + 1, l, (l + r) / 2);
        // 右の子
        Monoid c2 = query(a, b, 2 * k + 2, (l + r) / 2, r);
        return operation(c1, c2);
    }
};

void Main() {
    int N, M;
    cin >> N >> M;
    vector<pair<ll, P>> items(M);
    rep(i, M) {
        ll l, r, c;
        cin >> l >> r >> c;
        l--;
        r--;
        items[i] = make_pair(l, make_pair(r, c));
    }
    sort(items.begin(), items.end());
    vector<ll> dp(N, INF);
    dp[0] = 0;
    SegmentTree<ll> segmentTree(N, INF, [](ll a, ll b) { return min(a, b); }, [](ll a, ll b) { return b; });
    segmentTree.update(0, 0);
    rep(i, M) {
        ll l = items[i].first;
        ll r = items[i].second.first;
        ll c = items[i].second.second;
        ll tmp = segmentTree.query(l, r + 1);
        dp[r] = min(dp[r], tmp + c);
        segmentTree.update(r, dp[r]);
    }
    ll ans = dp[N - 1];
    if (ans == INF) {
        cout << -1 << '\n';
    } else {
        cout << ans << '\n';
    }
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
    Main();
    return 0;
}


Measured memory_bytes: 5376