// daughter of 671D codeforces
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 1e6 + 1;

struct line {
	int l, r, w;
	bool operator < (const line &o) {
		return r < o.r;
	}
} lie[N];

int n, m;
ll dp[N];
vector<int> cur[N];

int main() {
	cin.tie(0)->sync_with_stdio(0);
	cin >> n >> m;
	for (int i = 1, l, r, w; i <= m; i++) {
		cin >> l >> r >> w;
		lie[i] = {l, r, w};
		cur[l].push_back(i);
	}
	memset(dp, 0x3f3f3f3f, sizeof dp);
	set<pair<ll, int>> active;
	ll ans = LLONG_MAX;
	lie[0] = {1, 1, 0};
	active.insert({0, 0});
	for (int i = 1; i <= n; i++) {
		while (!active.empty() && lie[active.begin()->second].r < i) {
			active.erase(active.begin());
		}
		for (auto& id : cur[i]) {
			if (active.empty()) {
				cout << -1;
				return 0;
			}
			auto t = active.begin();
			dp[id] = min(dp[id], t->first + lie[id].w);
			if (lie[id].r == n) {
				ans = min(ans, dp[id]);
			}
		}
		for (auto& id : cur[i]) {
			active.insert({dp[id], id});
		}
	}
	if (ans == LLONG_MAX) cout << -1;
	else cout << ans;
}