#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <vector>
#define PII pair<int,int>
#define MP make_pair
#define fir first
#define sec second
#define PB push_back
#define ll long long
using namespace std;
template <class T>
inline void rd(T &x) {
	x=0; char c=getchar(); int f=1;
	while(!isdigit(c)) { if(c=='-') f=-1; c=getchar(); }
	while(isdigit(c)) x=x*10-'0'+c,c=getchar(); x*=f;
}
const int N=1e5+10;
const ll inf=1e18;
int n,m;
ll c[N];
void add(int i,ll t) { for(;i;i-=i&-i) c[i]=min(c[i],t); }
ll query(int i) { ll ans=inf; for(;i<=n;i+=i&-i) ans=min(ans,c[i]); return ans; }
vector<PII > G[N];
ll f[N];
int main() {
	rd(n),rd(m);
	for(int l,r,c,i=1;i<=m;++i) {
		rd(l),rd(r),rd(c);
		G[r].PB(MP(l,c));
	}
	memset(f,0x3f,sizeof(f));
	memset(c,0x3f,sizeof(c));
	f[1]=0,add(1,0);
	for(int i=2;i<=n;++i) {
		for(int j=0;j<G[i].size();++j)
			f[i]=min(f[i],query(G[i][j].fir)+G[i][j].sec);
		add(i,f[i]);
	}
	if(f[n]>1e16) printf("-1");
	else printf("%lld\n",f[n]);
	return 0;
}