#include <bits/stdc++.h>
#define lsb(x) (x & (-x))
#define ll long long
#define ull unsigned long long
#define uint unsigned int



#if 1
const int MOD = 998244353;

inline int lgput(int a, int b) {
    int ans = 1;
    while(b > 0) {
        if(b & 1) ans = (1LL * ans * a) % MOD;
        b >>= 1;
        a = (1LL * a * a) % MOD;
    }
    return ans;
}

inline void mod(int &x) {
    if(x >= MOD)
        x -= MOD;
}

inline void add(int &x, int y) {
    x += y;
    mod(x);
}

inline void sub(int &x, int y) {
    x += MOD - y;
    mod(x);
}

inline void mul(int &x, int y) {
    x = (1LL * x * y) % MOD;
}

inline int inv(int x) {
    return lgput(x, MOD - 2);
}
#endif

#if 0
int fact[], invfact[];

inline void prec(int n) {
    fact[0] = 1;
    for(int i = 1; i <= n; i++) {
        fact[i] = (1LL * fact[i - 1] * i) % MOD;
    }
    invfact[n] = lgput(fact[n], MOD - 2);
    for(int i = n - 1; i >= 0; i--) {
        invfact[i] = (1LL * invfact[i + 1] * (i + 1)) % MOD;
    }
}

inline int comb(int n, int k) {
    if(n < k) return 0;
    return (1LL * fact[n] * (1LL * invfact[k] * invfact[n - k] % MOD)) % MOD;
}
#endif

using namespace std;

const ll INF = 1e18;

struct Node {
    ll mn;
    ll lazy;
    Node() {
        mn = lazy = INF;
    }
};

class Lazy_SegTree {
public:

    vector <Node> aint;
    int n;

    inline void init(int _n) {
        n = _n;
        aint.resize(4 * n + 1);
    }

    inline void update(int l, int r, Node cur) {
        update(1, 1, n, l, r, cur);
    }

    inline Node query(int l, int r) {
        return query(1, 1, n, l, r);
    }

private:

    inline void push(int nod) {
        aint[2 * nod].lazy = min(aint[nod].lazy, aint[2 * nod].lazy);
        aint[2 * nod + 1].lazy = min(aint[nod].lazy, aint[2 * nod + 1].lazy);
        aint[nod].mn = min(aint[nod].mn, aint[nod].lazy);
        aint[nod].lazy = INF;
    }

    void update(int nod, int left, int right, int l, int r, Node cur) {
        if(l <= left && right <= r) {
            aint[nod].lazy = min(aint[nod].lazy, cur.lazy);
            return ;
        }
        push(nod);
        int mid = (left + right) / 2;

        if(l <= mid) update(2 * nod, left, mid, l, r, cur);
        if(mid < r) update(2 * nod + 1, mid + 1, right, l, r, cur);
    }

    Node query(int nod, int left, int right, int l, int r) {
        if(l <= left && right <= r) {
            return aint[nod];
        }
        push(nod);
        int mid = (left + right) / 2;
        Node ans;

        if(l <= mid) ans = query(2 * nod, left, mid, l, r);
        if(mid < r) ans = query(2 * nod + 1, mid + 1, right, l, r);

        return ans;
    }
};

int main() {
#ifdef HOME
    ifstream cin("A.in");
    ofstream cout("A.out");
#endif
    int i, n, m;
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);

    cin >> n >> m;

    vector < vector < pair <int, int> > > segm(n + 1);
    for(i = 1; i <= m; i++) {
        int l, r, c;
        cin >> l >> r >> c;
        segm[l].push_back({r, c});
    }

    Lazy_SegTree st; st.init(n);
    vector <ll> dst(n + 1, INF);
    dst[1] = 0;

    for(i = 1; i <= n; i++) {
        if(i > 1) {
            Node cur = st.query(i, i);
            dst[i] = min(cur.lazy, cur.mn);
        }

        for(auto it : segm[i]) {
            Node cur; cur.lazy = min(INF, dst[i] + it.second);
            st.update(i, it.first, cur);
        }
    }

    if(dst[n] == INF) dst[n] = -1;
    cout << dst[n];

    return 0;
}


Measured memory_bytes: 12544