#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < n; i++)
using namespace std;
typedef long long ll;
typedef pair<ll,ll> P;
const int MAX_N = 100005;
const ll INF = 1e15;

struct edge
{
	int to,cost;
};

vector<edge> G[MAX_N];
ll d[MAX_N];

void init()
{
	rep(i,MAX_N)
	{
		d[i] = INF;
	}
}

int main()
{
	init();
	int N,M;
	edge e;
	cin >> N >> M;
	rep(i,N-1)
	{
		e.to = i;
		e.cost = 0;
		G[i+1].push_back(e);
	}
	rep(i,M)
	{
		int l,r,c;
		cin >> l >> r >> c;
		e.to = r-1;
		e.cost = c;
		G[l-1].push_back(e);
	}

	priority_queue<P, vector<P>, greater<P> > Q;
	d[0] = 0;
	Q.push(P(0,0));
	while(!Q.empty())
	{
		P p = Q.top();
		Q.pop();
		int v = p.second;
		if (d[v] < p.first) continue;
		for (int i = 0; i < G[v].size(); ++i)
		{
			edge e = G[v][i];
			if (d[e.to] > d[v] + e.cost)
			{
				d[e.to] = d[v] + e.cost;
				Q.push(P(d[e.to],e.to));
			}
		}
	}

	if (d[N-1] == INF) cout << "-1" << endl;
	else cout << d[N-1] << endl;
	return 0;
}