#include <map>
#include <set>
#include <vector>
#include <algorithm>
#include <iostream>
#include <bitset>
#include <cassert>
#include <queue>
#include <random>
#include <stack>
#include <iomanip>
#include <functional>

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

template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
    return '"' + s + '"';
}

string to_string(const char* s) {
    return to_string((string) s);
}

string to_string(bool b) {
    return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
    bool first = true;
    string res = "{";
    for (int i = 0; i < static_cast<int>(v.size()); i++) {
        if (!first) {
            res += ", ";
        }
        first = false;
        res += to_string(v[i]);
    }
    res += "}";
    return res;
}

template <size_t N>
string to_string(bitset<N> v) {
    string res = "";
    for (size_t i = 0; i < N; i++) {
        res += static_cast<char>('0' + v[i]);
    }
    return res;
}

template <typename A>
string to_string(A v) {
    bool first = true;
    string res = "{";
    for (const auto &x : v) {
        if (!first) {
            res += ", ";
        }
        first = false;
        res += to_string(x);
    }
    res += "}";
    return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
    return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
    return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
    return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << " " << to_string(H);
    debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

using Int = unsigned int;
using llong = long long;
using Llong = unsigned long long;
using ldouble = long double;
using intV = vector<int>;
using llongV = vector<long long>;
using llongVV = vector<vector<long long>>;
using Graph = vector<vector<long long>>;
using costGraph = vector<vector<pair<long long, long long>>>;

struct Edge {
    long long from;
    long long to;
    long long cost;
};

const llong MOD = 1e9 + 7;
const llong INF = 1e17;

#define FOR(i, n) for (llong i = 0LL; i < llong(n); i++)
#define FORS(i, a, b) for (llong i = llong(a); i < llong(b); i++)
#define sup(vec, a) upper_bound(vec.begin(), vec.end(), a) - vec.begin()
#define inf(vec, a) lower_bound(vec.begin(), vec.end(), a) - vec.begin()

// 0-indexed, [l, r)
template<class T = long long>
class SegmentTree {
    public :
        vector<T> __data;
        T __size;
        T initValue;
        function<T(T, T)> cmpFunc;

        SegmentTree (T size, T initValue = 100000000000000000LL, function<T(T, T)> cmpFunc = [](T l, T r){return min(l, r);}) {
            this->init(size, initValue, cmpFunc);
        };

        void init (T size, T initValue, function<T(T, T)> cmpFunc) {
            // 完全二分木にする
            T binSize = 0;
            while((1 << binSize) < size) {
                binSize++;
            }
            this->__size = (1 << binSize);
            this->initValue = initValue;
            this->cmpFunc = cmpFunc;

            this->__data.resize(this->__size * 2);
            for(T i = 0; i < this->__size * 2; i++) {
                this->__data[i] = initValue;
            }
        };

        /* メソッド */
        void update(T index, T value) {
            index += this->__size - 1;
            this->__data[index] = value;
            while(index > 0) {
                index = (index - 1) / 2;
                this->__data[index] = this->cmpFunc(this->__data[index * 2 + 1], this->__data[index * 2 + 2]);
            }
        };

        T __query(T qLeft, T qRight, T node, T left, T right) {
            if(right <= qLeft || qRight <= left) return initValue;
            if(qLeft <= left && right <= qRight) return this->__data[node];

            T leftVal = this->__query(qLeft, qRight, node * 2 + 1, left, (left + right) / 2);
            T rightVal = this->__query(qLeft, qRight, node * 2 + 2, (left + right) / 2, right);
            return this->cmpFunc(leftVal, rightVal);
        }

        // wrapper
        T query(T left, T right) {
            return this->__query(left, right, 0, 0, this->__size);
        };
};

int main(void) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    llong N, M;
    cin >> N >> M;

    SegmentTree<llong> tree(N + 1);
    tree.update(1, 0);
    vector<tuple<llong, llong, llong>> edges(M);

    FOR(i, M) cin >> get<0>(edges[i]) >> get<1>(edges[i]) >> get<2>(edges[i]);
    sort(edges.begin(), edges.end());

    FOR(i, 10) {
        for(auto a : edges) {
            llong L, R, C;
            tie(L, R, C) = a;
            llong cost = tree.query(L, R + 1) + C;
            tree.update(R, min(cost, tree.query(R, R + 1)));
        }
    }

    llong ans = tree.query(N, N + 1);
    if(ans == INF) ans = -1;
    cout << ans << endl;

    return 0;
}

Measured memory_bytes: 4608