#include <bits/stdc++.h>
using namespace std;
using LL = long long;

#define fs first
#define sc second

const LL MOD = 1e9+7;
const double EPS = 1e-10;

template<class T>bool chmax(T &a, const T &b) {
    if (a < b) { a = b; return true; }
    return false;
}
template<class T>bool chmin(T &a, const T &b) {
    if (b < a) { a = b; return true; }
    return false;
}

// std::vector Declaration
template<typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts) {
    return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

// std::vector Declaration and Initialization
template<typename T>
vector<T> make_vector(size_t a, T x) { return vector<T>(a, x); }
template<typename T, typename U, typename... Ts>
auto make_vector(size_t a, U b, Ts... ts) {
    return vector<decltype(make_vector<T>(b,ts...))>(a,make_vector<T>(b,ts...));
}

// std::vector Input
template<typename T>
istream& operator>>(istream& is, vector<T>& v) {
    for (auto &e : v) {
        is >> e;
    }
    return is;
}

// std::vector Debug
template<typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::array Debug
template<typename T, size_t n>
ostream& operator<<(ostream& os, const array<T, n>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::deque Debug
template<typename T>
ostream& operator<<(ostream& os, const deque<T>& d) {
    os << "[";
    bool a = 1;
    for (auto e : d) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::pair Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
    os << "(" << p.first << " " << p.second << ")";
    return os;
}

// std::set Debug
template<typename T>
ostream& operator<<(ostream& os, const set<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::multiset Debug
template<typename T>
ostream& operator<<(ostream& os, const multiset<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::map Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const map<T, U>& mp) {
    os << "{";
    bool a = 1;
    for (auto e : mp) {
        os << (a ? "" : " ");
        os << e.first << ":" << e.second;
        a = 0;
    }
    os << "}";
    return os;
}

// std::tuple Debug
template<int N, class Tuple>
void out(ostream& os, const Tuple& t){}
template<int N, class Tuple, class H, class ...Ts>
void out(ostream& os, const Tuple& t) {
    if (N) os << " ";
    os << get<N>(t);
    out<N+1,Tuple,Ts...>(os, t);
}
template<class ...Ts>
ostream& operator<<(ostream& os, const tuple<Ts...>& t) {
    os << "(";
    out<0,tuple<Ts...>,Ts...>(os, t);
    os << ")";
    return os;
}

// Debug
#define DUMP(x) cerr<<#x<<" = "<<(x)<<endl

// Weighted edge
template<typename T>
struct edge {
    int src, to;
    T cost;

    edge(int to, T cost) : src(-1), to(to), cost(cost) {}
    edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

    edge &operator=(const int &x) {
        to = x;
        return *this;
    }

    operator int() const { return to; }

    friend ostream& operator<<(ostream& os, const edge& e) {
        return os << "(" << e.src << "->" << e.to << ":" << e.cost << ")";
    }
};

template<typename T>
using Edges = vector<edge<T>>;
template<typename T>
using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template<typename T>
using MatrixGraph = vector<vector<T>>;

template<typename Monoid>
struct SegmentTree {
    using F = function<Monoid(Monoid,Monoid)>;
    int sz;
    vector< Monoid > data;
    const F f;
    const Monoid M1;
    SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1) {
        sz = 1;
        while(sz < n) sz <<= 1;
        data.assign(2 * sz, M1);
    }
    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[2 * k + 0], data[2 * k + 1]);
        }
    }
    void update(int k, const Monoid &x) {
        k += sz;
        data[k] = x;
        while(k >>= 1) {
            data[k] = f(data[2 * k + 0], data[2 * k + 1]);
        }
    }
    void add(int k, const Monoid &x) {
        update(k, data[k+sz] + x);
    }
    Monoid query(int a, int b, int k, int l, int r) {
        if (r <= a || b <= l) {
            return M1;
        } else if (a <= l && r <= b) {
            return data[k];
        } else {
            return f(query(a, b, 2 * k + 0, l, (l + r) / 2),
                    query(a, b, 2 * k + 1, (l + r) / 2, r));
        }
    }
    Monoid query(int a, int b) {
        // return f[a, b).
        return query(a, b, 1, 0, sz);
    }
};


int main()
{
    int n, m; cin >> n >> m;
    vector<int> l(m), r(m), c(m);
    for (int i = 0; i < m; ++i) {
        cin >> l[i] >> r[i] >> c[i];
        --l[i], --r[i];
    }

    const LL INF = numeric_limits<LL>::max();
    auto rmq = SegmentTree<LL>(n, [](LL a, LL b){return min(a,b);}, INF);
    rmq.set(0, 0);
    rmq.build();

    vector<int> ord(m);
    iota(ord.begin(), ord.end(), 0);
    sort(ord.begin(), ord.end(), [&](int i, int j) {
            return r[i] < r[j];
            });


    for (int i = 0; i < m; ++i) {
        int id = ord[i];
        LL mi = rmq.query(l[id], r[id]);
        if (mi == INF) {
            continue;
        }

        LL prev = rmq.query(r[id], r[id]+1);
        rmq.update(r[id], min(prev, mi + c[id]));
    }

    LL ans = rmq.query(n-1, n);
    if (ans == INF) {
        ans = -1;
    }
    cout << ans << endl;


    return 0;
}


Measured memory_bytes: 3840