#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
template <class T> using treap = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
using Int = long long;
#define REP(i, n) for (long long i = 0, max_i = (n); i < max_i; i++)
#define REPI(i, a, b) for (long long i = (a), max_i = (b); i < max_i; i++)
#define ALL(obj) begin(obj), end(obj)
#define RALL(obj) rbegin(obj), rend(obj)
#define fi first
#define se second
using ii = pair<Int, Int>;
vector<ii> dirs = {
    {1, 0}, {0, 1}, {-1, 0}, {0, -1},  // 4方向
    {1, 1}, {-1, 1}, {-1, -1}, {1, -1},  // 斜め
    {0, 0},  // 自身
};
Int get_msb(long long x) {
    assert(x != 0);
    return 63 - __builtin_clzll(x);
}
Int get_lsb(long long x) {
    assert(x != 0);
    return __builtin_ctzll(x);
}
template <class T> inline bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } return false; }
template <class T> inline bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; } return false; }
template <class T, class S> vector<T> make_vec(size_t n, S x) { return vector<T>(n, x); }
template <class T, class... Ts> auto make_vec(size_t n, Ts... ts) { return vector<decltype(make_vec<T>(ts...))>(n, make_vec<T>(ts...)); }

// debug
template <class T> ostream& operator<<(ostream& s, const vector<T>& d) { REP (i, d.size()) s << d[i] << (i == d.size() - 1 ? "" : " "); return s; }
template <class T> ostream& operator<<(ostream& s, const vector<vector<T>>& d) { REP (i, d.size()) s << d[i] << (i == d.size() - 1 ? "" : "\n"); return s; }
template <class T, class S> ostream& operator<<(ostream& s, const pair<T, S>& p) { s << "{" << p.first << ", " << p.second << "}"; return s; }
template <class T> ostream& operator<<(ostream& s, const set<T> m) { for (auto it = m.begin(); it != m.end(); it++) { s << *it << (next(it) == m.end() ? "" : "\n"); } return s; }
template <class T> ostream& operator<<(ostream& s, const multiset<T> m) { for (auto it = m.begin(); it != m.end(); it++) { s << *it << (next(it) == m.end() ? "" : "\n"); } return s; }
template <class T, class S> ostream& operator<<(ostream& s, const map<T, S> m) { for (auto it = m.begin(); it != m.end(); it++) { s << *it << (next(it) == m.end() ? "" : "\n"); } return s; }
template <class T, class S> ostream& operator<<(ostream& s, const unordered_map<T, S> m) { for (auto it = m.begin(); it != m.end(); it++) { s << *it << (next(it) == m.end() ? "" : "\n"); } return s; }
#ifdef _MY_DEBUG
    #define dump(...) cerr << "/* " << #__VA_ARGS__ << " :[" << __LINE__ << ":" << __FUNCTION__ << "]" << endl, dump_func(__VA_ARGS__), cerr << "*/\n\n";
#else
    #define dump(...)
    #define endl "\n"
#endif
void dump_func() { cerr << endl; }
template <class Head, class... Tail> void dump_func(Head&& h, Tail&&... t) { cerr << h << (sizeof...(Tail) == 0 ? "" : ", "), dump_func(forward<Tail>(t)...); }

struct FastIO { FastIO() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20); } } fast_io;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
constexpr Int MOD = 1000000007;
// **********************************************************************************************
// **********************************************************************************************
// **********************************************************************************************
// **************************************** TEMPLATE END ****************************************
// **********************************************************************************************
// **********************************************************************************************
// **********************************************************************************************

template <class T, class F>
struct SegmentTree {
    const F op;
    const T e;
    SegmentTree(F op_, T e_) : op(op_), e(e_) {}
    int n;
    vector<T> t;
    void set_by_unit(int n_) {
        t.clear();
        n = n_;
        t.resize(2 * n, e);
    }
    void set_by_vector(const vector<T>& a) {
        t.clear();
        n = a.size();
        t.resize(2 * n, e);
        for (int i = 0; i < n; i++) t[i + n] = a[i];
        build();
    }
    void build() {
        for (int i = n - 1; i; --i) t[i] = op(t[2 * i], t[2 * i + 1]);
    }
    T& operator[](int i) { return t[i + n]; }
    // [l, r)
    T query(int l, int r) const {
        assert(0 <= l && l <= r && r <= n);
        T resl = e, resr = e;
        for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
            if (l & 1) resl = op(resl, t[l++]);
            if (r & 1) resr = op(t[--r], resr);
        }
        return op(resl, resr);
    }
    // iをaに変更
    void update(int i, const T& a) {
        assert(0 <= i && i < n);
        t[i += n] = a;
        while (i >>= 1) t[i] = op(t[2 * i], t[2 * i + 1]);
    }
};
template <class T, class F>
auto make_segment_tree(F op, T e) {
    return SegmentTree<T, F>(op, e);
}
// example
//    auto seg_mi = make_segment_tree<Int>([](Int a, Int b) { return min(a, b); }, 1e18);
//    auto seg_ma = make_segment_tree<Int>([](Int a, Int b) { return max(a, b); }, -1e18);

signed main() {
    Int n, m; cin >> n >> m;
    vector<Int> l(m), r(m), c(m);
    REP (i, m) cin >> l[i] >> r[i] >> c[i], l[i]--, r[i]--;
    vector<Int> ord(m);
    REP (i, m) ord[i] = i;
    sort(ALL(ord), [&](Int i, Int j) { return r[i] < r[j]; });
    dump(ord);
    constexpr Int INF = 1e18;
    auto seg_mi = make_segment_tree<Int>([](Int a, Int b) { return min(a, b); }, INF);
    seg_mi.set_by_unit(n);
    seg_mi.update(0, 0);
    REP (k, m) {
        Int i = ord[k];
        Int mi = seg_mi.query(l[i], r[i] + 1);
        dump(i, mi, mi + c[i], r[i]);
        seg_mi.update(r[i], min(seg_mi[r[i]], mi + c[i]));
    }
    REP (i, n) dump(seg_mi[i]);
    cout << (seg_mi[n - 1] == INF ? -1 : seg_mi[n - 1]) << endl;
}


Measured memory_bytes: 4864