#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using P = pair<ll, int>;

// 参考: http://kazuma8128.hatenablog.com/entry/2017/12/29/081929
template <class MonoidOp>
struct LazySegmentTree {
    using Tm = typename MonoidOp::Tm;
    using To = typename MonoidOp::To;

    int n;
    int h;
    vector<Tm> data;
    vector<To> lazy;

    LazySegmentTree(int size, Tm initial_data_value = MonoidOp::unit()) {
        n = 1;
        h = 0;
        while (n < size) {
            h++;
            n <<= 1;
        }

        data.assign((n << 1), initial_data_value);
        lazy.assign((n << 1), MonoidOp::op_unit());

        if (initial_data_value != MonoidOp::unit()) {
            for (int i = n - 1; i > 0; i--) data[i] = MonoidOp::merge(data[(i << 1)], data[(i << 1) + 1]);
        }
    }

    LazySegmentTree(const vector<Tm>& v) {
        int size = v.size();
        n = 1;
        h = 0;
        while (n < size) {
            h++;
            n <<= 1;
        }

        data.assign((n << 1), MonoidOp::unit());
        lazy.assign((n << 1), MonoidOp::op_unit());

        for (int i = 0; i < size; i++) data[i + n] = v[i];
        for (int i = n - 1; i > 0; i--) data[i] = MonoidOp::merge(data[(i << 1)], data[(i << 1) + 1]);
    }

    // https://komiyam.hatenadiary.org/entry/20131202/1385992406
    inline int getSegLen(int k) {
        return n / (1 << (31 - __builtin_clz(k)));
    }

    Tm getLeaf(int k) {
        return query(k, k + 1);
    }

    void push(int k) {
        if (lazy[k] == MonoidOp::op_unit()) return;
        int seg_len = getSegLen(k);
        data[k] = MonoidOp::apply(data[k], lazy[k], seg_len);
        if (seg_len > 1) {
            lazy[(k << 1)] = MonoidOp::op_merge(lazy[(k << 1)], lazy[k]);
            lazy[(k << 1) + 1] = MonoidOp::op_merge(lazy[(k << 1) + 1], lazy[k]);
        }
        lazy[k] = MonoidOp::op_unit();
    }

    void update(int k) {
        Tm vl = MonoidOp::apply(data[(k << 1)], lazy[(k << 1)], getSegLen(k << 1));
        Tm vr = MonoidOp::apply(data[(k << 1) + 1], lazy[(k << 1) + 1], getSegLen((k << 1) + 1));
        data[k] = MonoidOp::merge(vl, vr);
    }

    //区間[a, b)に対する更新
    void update(int a, int b, To x) {
        a += n, b += n - 1;
        for (int i = h; i > 0; i--) {
            push(a >> i);
            push(b >> i);
        }

        for (int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
            if (l & 1) lazy[l] = MonoidOp::op_merge(lazy[l], x), l++;
            if (r & 1) --r, lazy[r] = MonoidOp::op_merge(lazy[r], x);
        }

        while (a >>= 1, b >>= 1, a) {
            if (lazy[a] == MonoidOp::op_unit()) update(a);
            if (lazy[b] == MonoidOp::op_unit()) update(b);
        }
    }

    //区間[a, b)に対するクエリに答える
    Tm query(int a, int b) {
        a += n, b += n - 1;
        for (int i = h; i > 0; i--) {
            push(a >> i);
            push(b >> i);
        }

        Tm vl = MonoidOp::unit(), vr = MonoidOp::unit();
        for (int l = a, r = b + 1; l < r; l >>= 1, r >>= 1) {
            if (l & 1) {
                vl = MonoidOp::merge(vl, MonoidOp::apply(data[l], lazy[l], getSegLen(l)));
                l++;
            }

            if (r & 1) {
                --r;
                vr = MonoidOp::merge(MonoidOp::apply(data[r], lazy[r], getSegLen(r)), vr);
            }
        }
        return MonoidOp::merge(vl, vr);
    }
};

// 以下、MonoidOpの例
template<class U = ll, class V = U>
struct RangeSumAdd {
    using Tm = U;
    using To = V;
    static Tm merge(Tm x, Tm y) { return x + y; }
    static To op_merge(To x, To y) { return x + y; }
    static Tm apply(Tm vm, To vo, int seg_len) { return vm + vo * seg_len; }
    static constexpr Tm unit() { return Tm(0); }
    static constexpr To op_unit() { return To(0); }
};

template<class U = ll, class V = U>
struct RangeMinUpdate {
    using Tm = U;
    using To = V;
    static Tm merge(Tm x, Tm y) { return min(x, y); }
    static To op_merge(To x, To y) { return y; }
    static Tm apply(Tm vm, To vo, int seg_len) { return vo == op_unit() ? vm : vo; }
    static constexpr Tm unit() { return numeric_limits<Tm>::max(); }
    static constexpr To op_unit() { return numeric_limits<To>::max(); }
};

// 作用素の初期値は更新クエリで与えられる値の定義域の範囲外の値にする
template<class U = ll, class V = U>
struct RangeSumUpdate {
    using Tm = U;
    using To = V;
    static Tm merge(Tm x, Tm y) { return x + y; }
    static To op_merge(To x, To y) { return y; }
    static Tm apply(Tm vm, To vo, int seg_len) { return vo == op_unit() ? vm : vo * seg_len; }
    static constexpr Tm unit() { return Tm(0); }
    static constexpr To op_unit() { return numeric_limits<To>::min(); }
};

// 初期値 != 単位元 の場合に注意
template<class U = ll, class V = U>
struct RangeMinAdd {
    using Tm = U;
    using To = V;
    static Tm merge(Tm x, Tm y) { return min(x, y); }
    static To op_merge(To x, To y) { return x + y; }
    static Tm apply(Tm vm, To vo, int seg_len) { return vm + vo; }
    static constexpr Tm unit() { return numeric_limits<Tm>::max(); }
    static constexpr To op_unit() { return To(0); }
};

constexpr ll INF = 1e16;

template<class U = ll, class V = U>
struct RangeMinChmin {
    using Tm = U;
    using To = V;
    static Tm merge(Tm x, Tm y) { return min(x, y); }
    static To op_merge(To target, To x) { return min(target, x); }
    static Tm apply(Tm target, To x, int seg_len) { return min(target, x); }
    static constexpr Tm unit() { return Tm(INF); }
    static constexpr To op_unit() { return To(INF); }
};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    vector< vector<P> > g(n);
    for (int i = 0; i < m; i++) {
        int l, r;
        ll c;
        cin >> l >> r >> c;
        l--; r--;
        g[l].emplace_back(c, r);
    }

    for (int i = 0; i < n; i++) {
        sort(g[i].begin(), g[i].end());
    }

    LazySegmentTree< RangeMinChmin<> > st(n);
    st.update(0, 1, 0);
    for (int i = 0; i < n; i++) {
        ll cost = st.getLeaf(i);
        for (P& p : g[i]) {
            st.update(i, p.second + 1, cost + p.first);
        }
    }
    ll ans = st.getLeaf(n - 1);
    cout << (ans >= INF ? -1 : ans) << "\n";
    return 0;
}

Measured memory_bytes: 9728