#include <bits/stdc++.h>
using namespace std;
//#define cerr if (false) cerr
#define db(x) cerr << #x << "=" << x << endl
#define db2(x, y) cerr << #x << "=" << x << "," << #y << "=" << y << endl
#define db3(x, y, z) cerr << #x << "=" << x << "," << #y << "=" << y << "," << #z << "=" << z << endl
#define dbv(v) cerr << #v << "="; for (auto _x : v) cerr << _x << ", "; cerr << endl
#define dba(a, n) cerr << #a << "="; for (int _i = 0; _i < (n); ++_i) cerr << a[_i] << ", "; cerr << endl
template <typename A, typename B>
ostream& operator<<(ostream& os, const pair<A, B>& x) {
	return os << "(" << x.first << "," << x.second << ")";
}
typedef long long ll;
typedef long double ld;
const ll INF = 1LL << 60;
struct SegTree {
	struct Node {
		ll x = INF; // Set default value here
//		ll prop = INF;
		// Used for updates and propagation.
		void apply(int s, int e, ll v) {
			x = min(x, v);
//			prop = min(prop, v);
		}
	};
	inline Node combine(const Node& a, const Node& b) {
		Node res;
		res.x = min(a.x, b.x);
		return res;
	}
	inline void push(int sn, int s, int e) {
//		if (tree[sn].prop) {
//			int m = (s + e) >> 1, lsn = sn * 2, rsn = sn * 2 + 1;
//			tree[lsn].apply(s, m, tree[sn].prop);
//			tree[rsn].apply(m + 1, e, tree[sn].prop);
//			tree[sn].prop = 0;
//		}
	}
	int start, end;
	vector<Node> tree;
	void build(int sn, int s, int e) {
		if (s == e) {
			// initialize here, possibly.
			return;
		}
		int m = (s + e) >> 1, lsn = sn * 2, rsn = sn * 2 + 1;
		build(lsn, s, m);
		build(rsn, m + 1, e);
		tree[sn] = combine(tree[lsn], tree[rsn]);
	}
	template <typename T>
	void build(int sn, int s, int e, vector<T>& v) {
		if (s == e) {
			tree[sn].x = v[s];
			return;
		}
		int m = (s + e) >> 1, lsn = sn * 2, rsn = sn * 2 + 1;
		build(lsn, s, m, v);
		build(rsn, m + 1, e, v);
		tree[sn] = combine(tree[lsn], tree[rsn]);
	}
	template <typename... T>
	void update(int sn, int s, int e, int qs, int qe, const T&... v) {
		if (qs <= s && e <= qe) {
			tree[sn].apply(s, e, v...);
			return;
		}
		int m = (s + e) >> 1, lsn = sn * 2, rsn = sn * 2 + 1;
		push(sn, s, e);
		if (qs <= m) update(lsn, s, m, qs, qe, v...);
		if (qe > m) update(rsn, m + 1, e, qs, qe, v...);
//		tree[sn] = combine(tree[lsn], tree[rsn]);
	}
	Node query(int sn, int s, int e, int qs, int qe) {
		if (qs <= s && e <= qe) return tree[sn];
		int m = (s + e) >> 1, lsn = sn * 2, rsn = sn * 2 + 1;
		push(sn, s, e);
		if (qe <= m) return combine(tree[sn], query(lsn, s, m, qs, qe));
		if (qs > m) return combine(tree[sn], query(rsn, m + 1, e, qs, qe));
		return combine(query(lsn, s, m, qs, qe), query(rsn, m + 1, e, qs, qe));
	}
	void query(int sn, int s, int e, int qs, int qe, const function<void(Node&, int, int)>& f) {
		if (qs <= s && e <= qe) return f(tree[sn], s, e);
		int m = (s + e) >> 1, lsn = sn * 2, rsn = sn * 2 + 1;
		push(sn, s, e);
		if (qs <= m) query(lsn, s, m, qs, qe, f);
		if (qe > m) query(rsn, m + 1, e, qs, qe, f);
	}
	SegTree(int n) : SegTree(0, n - 1) {}
	SegTree(int _start, int _end) : start(_start), end(_end) {
		int n = end - start + 1;
		int maxs = n == 1 ? 2 : 1 << (33 - __builtin_clz(n - 1));
		tree.resize(maxs);
		build(1, start, end);
	}
	template <typename T>
	SegTree(vector<T>& v) {
		int n = v.size();
		int maxs = n == 1 ? 2 : 1 << (33 - __builtin_clz(n - 1));
		tree.resize(maxs);
		start = 0;
		end = n - 1;
		build(1, start, end, v);
	}
	Node query(int qs, int qe) {
		return query(1, start, end, qs, qe);
	}
	Node query(int p) {
		return query(1, start, end, p, p);
	}
	void query(int qs, int qe, const function<void(Node&, int, int)>& f) {
		if (qs > qe) return;
		query(1, start, end, qs, qe, f);
	}
	template <typename... T>
	void update(int qs, int qe, const T&... v) {
		update(1, start, end, qs, qe, v...);
	}
};
int main() {
	int n, m;
	scanf("%d%d", &n, &m);
	vector<vector<pair<int, int>>> E(n + 1);
	for (int i = 0; i < m; ++i) {
		int l, r, c;
		scanf("%d%d%d", &l, &r, &c);
		E[l].push_back({r, c});
	}
	SegTree st(n + 1);
	st.update(1, 1, 0);
	for (int i = 1; i <= n; ++i) {
		for (auto& e : E[i]) {
			int j = e.first, c = e.second;
			ll u = st.query(i).x;
			st.update(i + 1, j, u + c);
		}
	}
	ll ans = st.query(n).x;
	if (ans == INF) printf("-1\n");
	else printf("%lld\n", ans);
}


Measured memory_bytes: 7680