#include <bits/stdc++.h>
#define r(i,n) for(int i=0;i<n;i++)
#define int long long
using namespace std;
typedef pair<int,int>P;
typedef pair<P,int>P2;
#define F first
#define S second

struct RLQ{
  int n=(1<<17),data[(1<<18)];
  RLQ(){
    fill(data,data+(1<<18),1e17);
  }
  void update(int k,int x){
    k+=n-1;
    data[k]=x;
    while(k>0){
      k=(k-1)/2;
      data[k]=min(data[k*2+1],data[k*2+2]);
    }
  }
  int find(int a,int b,int k=0,int l=0,int r=(1<<17)){
    if(r<=a || b<=l)return 1e17;
    if(a<=l && r<=b)return data[k];
    int v1=find(a,b,k*2+1,l,(l+r)/2);
    int v2=find(a,b,k*2+2,(l+r)/2,r);
    return min(v1,v2);
  }
};

int n,m;

signed main(){
	RLQ R;
	vector<P2>v;
	cin>>n>>m;
	r(i,m){
		int a,b,c;
		cin>>a>>b>>c; a--; b--;
		v.push_back(P2(P(b,a),c));
	}
	sort(v.begin(),v.end());
	R.update(0,0);
	r(i,m){
		int l=v[i].F.S;
		int r=v[i].F.F;
		int c=v[i].S;
		int MIN = R.find(l,r)+c;
		int X=R.find(r,r+1);
		if(X>MIN){
			R.update(r,MIN);
		}
	}
	int A=R.find(n-1,n);
	if(A>1e16)cout<<-1<<endl;
	else cout<<A<<endl;
}
