#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T &a, T b)
{
    if (a < b)
    {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
inline bool chmin(T &a, T b)
{
    if (a > b)
    {
        a = b;
        return 1;
    }
    return 0;
}
typedef long long int ll;

#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
#define endl "\n"
const double EPS = 1e-7;
const int INF = 1 << 30;
const ll LLINF = 1LL << 60;
const double PI = acos(-1);
const int MOD = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

//-------------------------------------

template <typename T, typename E>
struct LazySegmentTree
{
    using F = function<T(T, T)>;
    using G = function<T(T, E)>;
    using H = function<E(E, E)>;
    using P = function<E(E, int)>;
    F f;
    G g;
    H h;
    P p;
    T d1;
    E d0;
    int n;
    vector<T> node;
    vector<E> lazy;

    LazySegmentTree(int sz, F f, G g, H h, T d1, E d0, vector<T> v = vector<T>(), P p = [](E a, int b) { return a; }) : f(f), g(g), h(h), d1(d1), d0(d0), p(p)
    {
        init(sz);
        if (sz == (int)v.size())
        {
            build(sz, v);
        }
    }

    void init(int sz)
    {
        n = 1;
        while (n < sz)
        {
            n *= 2;
        }
        node.clear();
        node.resize(2 * n - 1, d1);
        lazy.clear();
        lazy.resize(2 * n - 1, d0);
    }

    void build(int sz, vector<T> v)
    {
        for (int i = 0; i < sz; i++)
        {
            node[i + n - 1] = v[i];
        }
        for (int i = n - 2; i >= 0; i--)
        {
            node[i] = f(node[2 * i + 1], node[2 * i + 2]);
        }
    }

    inline void eval(int len, int now)
    {
        if (lazy[now] == d0)
        {
            return;
        }
        if (2 * now + 1 < 2 * n - 1)
        {
            lazy[2 * now + 1] = h(lazy[2 * now + 1], lazy[now]);
            lazy[2 * now + 2] = h(lazy[2 * now + 2], lazy[now]);
        }
        node[now] = g(node[now], p(lazy[now], len));
        lazy[now] = d0;
    }

    T update(int a, int b, E x, int now = 0, int l = 0, int r = -1)
    {
        if (r < 0)
        {
            r = n;
        }
        eval(r - l, now);
        if (r <= a || b <= l)
        {
            return node[now];
        }
        if (a <= l && r <= b)
        {
            lazy[now] = h(lazy[now], x);
            return g(node[now], p(lazy[now], r - l));
        }
        T vl = update(a, b, x, 2 * now + 1, l, (l + r) / 2);
        T vr = update(a, b, x, 2 * now + 2, (l + r) / 2, r);
        return node[now] = f(vl, vr);
    }

    T query(int a, int b, int now = 0, int l = 0, int r = -1)
    {
        if (r < 0)
        {
            r = n;
        }
        eval(r - l, now);
        if (r <= a || b <= l)
        {
            return d1;
        }
        if (a <= l && r <= b)
        {
            return node[now];
        }
        T vl = query(a, b, 2 * now + 1, l, (l + r) / 2);
        T vr = query(a, b, 2 * now + 2, (l + r) / 2, r);
        return f(vl, vr);
    }
};

using P = pair<ll, ll>;
using ppl = pair<P, ll>;

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll n, m;
    cin >> n >> m;
    vector<ppl> LR(m);
    for (ll i = 0; i < m; i++)
    {
        ll a, b, c;
        cin >> a >> b >> c;
        a--;
        b--;
        LR[i] = ppl(P(a, b), c);
    }
    sort(ALL(LR));
    auto f = [](ll a, ll b) { return min(a, b); };
    auto g = f;
    auto h = f;
    LazySegmentTree<ll, ll> dp(101010, f, g, h, LLINF, LLINF);
    dp.update(0, 1, 0);
    for (ll i = 0; i < m; i++)
    {
        ll l = LR[i].first.first;
        ll r = LR[i].first.second;
        ll c = LR[i].second;
        ll tmp = dp.query(l, l + 1);
        dp.update(l + 1, r + 1, tmp + c);
    }
    ll ans = dp.query(n - 1, n);
    cout << (ans == LLINF ? -1 : ans) << endl;
}

Measured memory_bytes: 6656