#include <bits/stdc++.h>
#define mmp(a,b) make_pair(a,b)
using namespace std;
typedef long long ll;
const int MN = 100100;
const ll INFL = 1LL<<61;
typedef vector<ll> vl;
typedef pair<ll,ll> pl;
typedef vector<pl> vpl;
bitset<MN> bs;
vpl st[MN];
vl ed[MN];
typedef vector<pl> vpl;
ll res[MN];
int main() {
	ios::sync_with_stdio(0);cin.tie(0);
	priority_queue<pl,vector<pl>,greater<pl> > pq;
	ll n,m;
	cin >> n >> m;
	fill(res,res+n,INFL);
	for(int i=0;i<m;i++) {
		ll a,b,c;
		cin >> a >> b >> c;
		a--;b--;
		st[a].push_back(mmp(i,c));
		ed[b+1].push_back(i);
	}
	pq.push({INFL,m});
	res[0] = 0;
	for(int i=0;i<n;i++) {
		for(int j=0;j<ed[i].size();j++) {
			bs.set(ed[i][j]);
		}
		while(bs[pq.top().second]) {
			pq.pop();
		}
		res[i] = min(res[i],pq.top().first);
		for(int j=0;j<st[i].size();j++) {
			ll id = st[i][j].first,co = st[i][j].second;
			ll psh = min(co+res[i],INFL);
			pq.push(mmp(psh,id));
		}
	}
	if(res[n-1] == INFL) {
		cout << -1 << '\n';
	} else {
		cout << res[n-1] << '\n';
	}
}
