#include <bits/stdc++.h>
#define MOD (ll)(1e9 + 7)
#define MAX (ll)(2e5 + 4)
#define INF (ll)(1e17)
using namespace std;
using ll =  long long;
using ld = long double;

vector<pair<ll, ll>> ve[MAX];

int main () {
	ll n, m;
	cin >> n >> m;
	for (ll i = 0; i < m; i++) {
		ll l, r, c;
		cin >> l >> r >> c;
		ve[l].push_back (make_pair (c, r));
	}
	ll ne = 1, dp[n + 1];
	for (int i = 0; i <= n; i++) {
		dp[i] = INF;			
	}
	dp[1] = 0;
	set <pair<ll, ll>> sp;
	ll co = 0;
	for (ll i = 1; i < n; i++) {
		for (auto x : ve[i]) {
			sp.insert (make_pair (dp[i] + x.first, x.second));
		}
		while (sp.size () && sp.begin () -> second <= i) {
			sp.erase (sp.begin ());
		}
		if (sp.size ()) {
			dp[i + 1] = min (dp[i + 1], sp.begin () -> first);
		}
		else {
			cout << "-1\n";
			return 0;
		}
	}
	cout << dp[n] << "\n";
}
		
		
		
