#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <complex>
#include <cstring>
using namespace std;

using ll = long long;
using P = pair<int, int>;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
// constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

template<typename Monoid, typename OperatorMonoid = Monoid>
struct LazySegmentTree{
    using F = function<Monoid(Monoid, Monoid)>;
    using G = function<Monoid(Monoid, OperatorMonoid)>;
    using H = function<OperatorMonoid(OperatorMonoid, OperatorMonoid)>;
    using C = function<OperatorMonoid(OperatorMonoid, int)>;

    int sz;
    vector<Monoid> data;
    vector<OperatorMonoid> lazy;
    const F f;
    const G g;
    const H h;
    const Monoid M1;            // モノイドの単位元
    const OperatorMonoid OM0;   // 作用素モノイドの単位元
    const C c;

    LazySegmentTree(int n, const F f, const G g, const H h, const Monoid &M1,
        const OperatorMonoid OM0, const C c = [](OperatorMonoid a, int b){return a;})
        : f(f), g(g), h(h), M1(M1), OM0(OM0), c(c)
    {
        sz = 1;
        while(sz < n)   sz <<= 1;
        data.assign(sz << 1, M1);
        lazy.assign(sz << 1, OM0);
    }

    void set(int k, const Monoid &x){
        data[k + sz] = x;
    }

    void build(){
        for(int k = sz - 1; k > 0; --k){
            data[k] = f(data[k << 1], data[k << 1 | 1]);
        }
    }

    void propagate(int k, int len){
        if(lazy[k] != OM0){
            if(k < sz){
                lazy[k << 1] = h(lazy[k << 1], lazy[k]);
                lazy[k << 1 | 1] = h(lazy[k << 1 | 1], lazy[k]);
            }
            data[k] = g(data[k], c(lazy[k], len));
            lazy[k] = OM0;
        }
    }

    Monoid update(int a, int b, const OperatorMonoid &x, int k = 1, int l = 0, int r = -1){
        if(r == -1)     r = sz;
        propagate(k, r - l);
        if(r <= a || b <= l)    return data[k];
        else if(a <= l && r <= b){
            lazy[k] = h(lazy[k], x);
            propagate(k, r - l);
            return data[k];
        }
        else{
            return data[k] = f(update(a, b, x, k << 1, l, (l + r) >> 1),
                                update(a, b, x, k << 1 | 1, (l + r) >> 1, r));
        }
    }

    Monoid query(int a, int b, int k = 1, int l = 0, int r = -1){
        if(r == -1)     r = sz;
        propagate(k, r - l);
        if(r <= a || b <= l)    return M1;
        else if(a <= l && r <= b)   return data[k];
        else{
            return f(query(a, b, k << 1, l, (l + r) >> 1),
                        query(a, b, k << 1 | 1, (l + r) >> 1, r));
        }
    }

    Monoid operator[](const int &k){
        return query(k, k + 1);
    }
};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N, M;
    cin >> N >> M;
    vector<tuple<int, int, int>> data;
    for(int i = 0; i < M; ++i){
        int L, R, C;
        cin >> L >> R >> C;
        data.emplace_back(L, R, C);
    }
    sort(data.begin(), data.end());

    constexpr ll inf = 1e+17;
    LazySegmentTree<ll> seg(N + 1, [](ll a, ll b){return min(a, b);},
                                    [](ll a, ll b){return min(a, b);},
                                    [](ll a, ll b){return min(a, b);},
                                    inf, inf);
    seg.update(1, 2, 0);
    for(auto& tup : data){
        int L, R, C;
        tie(L, R, C) = tup;
        ll min_val = seg.query(L, R + 1);
        seg.update(L, R + 1, min_val + C);
    }

    ll ans = seg[N];
    if(ans == inf)  ans = -1;
    cout << ans << endl;
}

Measured memory_bytes: 8740