//              +-- -- --++-- +-In the name of ALLAH-+ --++-- -- --+              \\

#include <bits/stdc++.h>

#define int ll

#define F first
#define S second
#define _sz(x) (int)x.size()
#define pb push_back

#define error(x) cout << #x << " = " << x << endl

using namespace std;
using ll = long long;
using ld = long double;
using pii = pair<int, int>;

const int N = 1e5 + 20, inf = 3e18 + 20;
int n, m;
vector <pii> g[N];

int dist[N];

int32_t main()
{
	ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);

	cin >> n >> m;

	for (int i = 0; i < n - 1; i++)
		g[i + 1].pb({i, 0});

	for (int i = 0, l, r, c; i < m; i++)
	{
		cin >> l >> r >> c;
		l--, r--;
	
		g[l].pb({r, c});
	}
	
	set <pii> st;
	for (int i = 0; i < n; i++) dist[i] = inf;
	
	dist[0] = 0;
	st.insert({0, 0});

	while (_sz(st))
	{
		int v = st.begin() -> S;
		st.erase(st.begin());

		for (pii p : g[v])
		{
			int u = p.F, w = p.S;

			if (dist[v] + w < dist[u])
			{
				st.erase({dist[u], u});
				dist[u] = dist[v] + w;
				st.insert({dist[u], u});
			}
		}
	}

	if (dist[n - 1] == inf) dist[n - 1] = -1;

	cout << dist[n - 1] << '\n';
}
