#include "bits/stdc++.h"
// Begin Header {{{
#define all(x)          (x).begin(), (x).end()
#define rep(i, n)       for (i64 i = 0, i##_limit = (n); i < i##_limit; ++i)
#define reps(i, s, t)   for (i64 i = (s), i##_limit = (t); i <= i##_limit; ++i)
#define repr(i, s, t)   for (i64 i = (s), i##_limit = (t); i >= i##_limit; --i)
#define var(Type, ...)  Type __VA_ARGS__; input(__VA_ARGS__)
#ifndef DBG
#define trace(...)
#endif
using namespace std;
using i64 = int_fast64_t;
using pii = pair<i64, i64>;
template <class T, class U> inline bool chmax(T &a, const U &b) { return b > a && (a = b, true); }
template <class T, class U> inline bool chmin(T &a, const U &b) { return b < a && (a = b, true); }
inline i64  sigma(i64 n)            { return (n * (n + 1) >> 1); }
inline i64  updiv(i64 a, i64 b)     { return (a + b - 1) / b; }
inline i64  sqr(i64 n)              { return n * n; }
inline string to_string(char c)     { return string(1, c); }
constexpr int INF  = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;

template <class T>
inline vector<T> make_v(const T &initValue, size_t sz) {
    return vector<T>(sz, initValue);
}
template <class T, class... Args>
inline auto make_v(const T &initValue, size_t sz, Args... args) {
    return vector<decltype(make_v<T>(initValue, args...))>(sz, make_v<T>(initValue, args...));
}

inline void input() {}
template <class Head, class... Tail>
inline void input(Head &head, Tail&... tail) { cin >> head; input(tail...); }

inline void print() { cout << "\n"; }
template <class Head, class... Tail>
inline void print(Head &&head, Tail&&... tail) {
    cout << head;
    if (sizeof...(tail)) cout << ' ';
    print(forward<Tail>(tail)...);
}

template <class T>
inline ostream& operator<< (ostream &out, const vector<T> &vec) {
    static constexpr const char *delim[] = { " ", "" };
    for (const auto &e : vec) out << e << delim[&e == &vec.back()];
    return out;
}

template <typename Func>
struct FixPoint : Func {
    inline constexpr FixPoint(Func &&f) noexcept : Func(forward<Func>(f)) {}

    template <typename... Args>
    inline decltype(auto) operator()(Args &&... args) const {
        return Func::operator()(*this, forward<Args>(args)...);
    }
};

template< typename Func >
inline decltype(auto) makeFixPoint(Func &&f) {
    return FixPoint< Func >{forward< Func >(f)};
}
// }}} End Header

// graph {{{
struct Edge { // {{{
    int src, to;
    int64_t cost = 0;

    inline Edge() noexcept {}
    inline Edge(int s,  int t, int64_t c = 1) noexcept : src(s), to(t), cost(c) {}

    inline bool operator<(const Edge &rhs) const noexcept { return cost < rhs.cost; }
    inline bool operator>(const Edge &rhs) const noexcept { return rhs < *this; }

    inline operator int() const noexcept { return to; }

    inline Edge& operator=(int rhs_to) noexcept { this->to = rhs_to; return *this; }
};
// }}}

class Graph : public vector<vector<Edge>> { // {{{
    using super = vector<vector<Edge>>;
public:
    Graph() noexcept {}
    Graph(int V) noexcept : super(V) {}

    Graph& addEdge(int from, int to, int64_t cost = 1) noexcept {
        super::operator[](from).emplace_back(from, to, cost);
        return *this;
    }

    Graph& addEdge(const Edge &e) noexcept {
        return addEdge(e.src, e.to, e.cost);
    }

    friend ostream& operator<<(ostream &out, const Graph &g) {
        for (int i = 0; i < g.size(); ++i) {
            out << '[' << setw(2) << i << ']';
            for (const auto &e : g[i]) {
                out << ' ' << setw(2) << e.to;
            }
            out << '\n';
        }
        return out;
    }
}; // }}}
// }}}

template<class Compare = less<>, int64_t INF = 0x3f3f3f3f3f3f3f3fLL>
vector<int64_t> dijkstra(const Graph &G, const vector<int> &startNodes) // {{{
{
    using pii = pair<int64_t, int>;
    const auto isBetter = Compare();

    vector<int64_t> dist(G.size(), INF);
    priority_queue<pii, vector<pii>, Compare> pq;

    for (const int startNode : startNodes) {
        dist[startNode] = 0LL;
        pq.emplace(-0LL, startNode);
    }

    while(!pq.empty()) {
        const int64_t nowCost = -(pq.top().first);
        const int     nowNode = pq.top().second;
        pq.pop();

        if (isBetter(dist[nowNode], nowCost)) continue;

        for (const auto &e : G[nowNode]) {
            const int64_t newCost = nowCost + e.cost;
            if (isBetter(newCost, dist[e.to])) {
                dist[e.to] = newCost;
                pq.emplace(-newCost, e.to);
            }
        }
    }

    return dist;
}
// }}}

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

    var(int, N, M);
    vector<int> L(M), R(M), C(M);

    int imos[100010] {};

    Graph G(N + 1);

    rep(i, M) {
        input(L[i], R[i], C[i]);
        ++imos[L[i]];
        --imos[R[i]];
        G.addEdge(L[i], R[i], C[i]);
        G.addEdge(R[i], L[i], C[i]);
    }

    rep(i, 100010 - 1) {
        imos[i + 1] += imos[i];
        if (imos[i] > 0) {
            G.addEdge(i+1, i, 0);
        }
    }

    const auto dist = dijkstra(G, {1});

    if (dist[N] >= LINF) {
        print(-1);
    } else {
        print(dist[N]);
    }

    return 0;
}


Measured memory_bytes: 14200