#include <bits/stdc++.h>
#define clr(x) memset(x,0,sizeof(x))
#define For(i,a,b) for (int i=(a);i<=(b);i++)
#define Fod(i,b,a) for (int i=(b);i>=(a);i--)
#define fi first
#define se second
#define kill _z_kill
#define y0 _z_y0
#define y1 _z_y1
#define x0 _z_x0
#define x1 _z_x1
#define pb(x) push_back(x)
#define mp(x,y) make_pair(x,y)
using namespace std;
typedef long long LL;
typedef unsigned long long ull;
typedef unsigned uint;
typedef long double LD;
typedef vector <int> vi;
typedef pair <int,int> pii;
void enable_comma(){}
string tostring(char c);
string tostring(LL x);
template <class A,class B> string tostring(pair <A,B> p);
template <class A> string tostring(vector <A> v);
string tostring(char c){
	string s="";
	s+=c;
	return s;
}
string tostring(string s){
	return "\""+s+"\"";
}
string tostring(char *c){
	return tostring((string)c);
}
string tostring(LL x){
	if (x<0)
		return "-"+tostring(-x);
	if (x>9)
		return tostring(x/10)+tostring(char('0'+x%10));
	else
		return tostring(char('0'+x));
}
string tostring(int x){
	return tostring((LL)x);
}
string tostring(ull x){
	if (x>9)
		return tostring((LL)(x/10))+tostring(char('0'+x%10));
	else
		return tostring(char('0'+x));
}
string tostring(uint x){
	return tostring((LL)x);
}
template <class A,class B> string tostring(pair <A,B> p){
	return "("+tostring(p.fi)+","+tostring(p.se)+")";
}
template <class A> string tostring(vector <A> v){
	string res="{";
	For(i,0,(int)v.size()-1){
		res+=tostring(v[i]);
		res+=i==(int)v.size()-1?"}":",";
	}
	return res;
}
template <class A> string tostring(A *a,int L,int R){
	string res="{";
	For(i,L,R){
		res+=tostring(a[i]);
		res+=i==R?"}":",";
	}
	return res;
}
string tostrings(){
	return "";
}
template <typename Head,typename... Tail>
string tostrings(Head H,Tail... T){
	return tostring(H)+" "+tostrings(T...);
}
#define User_Time ((double)clock()/CLOCKS_PER_SEC)
#ifdef zzd
	#define outval(x) cerr<<#x" = "<<tostring(x)<<endl
	#define outvals(...) cerr<<"["<<#__VA_ARGS__<<"]: "<<tostrings(__VA_ARGS__)<<endl
	#define outtag(x) cerr<<"--------------"#x"---------------"<<endl
	#define outsign(x) cerr<<"<"#x">"<<endl
	#define outarr(a,L,R) cerr<<#a"["<<(L)<<".."<<(R)<<"] = "<<tostring(a,L,R)<<endl
#else
	#define outval(x) enable_comma()
	#define outvals(...) enable_comma()
	#define outtag(x) enable_comma()
	#define outsign(x) enable_comma()
	#define outarr(a,L,R) enable_comma()
#endif
#ifdef ONLINE_JUDGE
	#ifdef assert
		#undef assert
	#endif
	#define assert(x) (!(x)?\
			cout<<"Assertion failed!"<<endl<<\
			"function: "<<__FUNCTION__<<endl<<\
			"line: "<<__LINE__<<endl<<\
			"expression: "<<#x<<endl,exit(3),0:1)
#endif
LL read(){
	LL x=0,f=0;
	char ch=getchar();
	while (!isdigit(ch))
		f=ch=='-',ch=getchar();
	while (isdigit(ch))
		x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
	return f?-x:x;
}
template <class T> void ckmax(T &x,const T y){
	if (x<y)
		x=y;
}
template <class T> void ckmin(T &x,const T y){
	if (x>y)
		x=y;
}
//int Pow(int x,int y){
//	int ans=1;
//	for (;y;y>>=1,x=(LL)x*x%mod)
//		if (y&1)
//			ans=(LL)ans*x%mod;
//	return ans;
//}
//void Add(int &x,int y){
//	if ((x+=y)>=mod)
//		x-=mod;
//}
//void Del(int &x,int y){
//	if ((x-=y)<0)
//		x+=mod;
//}
//int Add(int x){
//	return x>=mod?x-mod:x;
//}
//int Del(int x){
//	return x<0?x+mod:x;
//}
//int md(LL x){
//	return (x%mod+mod)%mod;
//}
const int N=100005*4;
const LL INF=1e18;
int n,m;
LL mi[N];
void update(int rt,int L,int R,int x,LL v){
	if (L==R){
		mi[rt]=v;
		return;
	}
	int mid=(L+R)>>1,ls=rt<<1,rs=ls|1;
	if (x<=mid)
		update(ls,L,mid,x,v);
	else
		update(rs,mid+1,R,x,v);
	mi[rt]=min(mi[ls],mi[rs]);
}
LL query(int rt,int L,int R,int xL,int xR){
	if (R<xL||L>xR)
		return INF;
	if (xL<=L&&R<=xR)
		return mi[rt];
	int mid=(L+R)>>1,ls=rt<<1,rs=ls|1;
	return min(query(ls,L,mid,xL,xR),query(rs,mid+1,R,xL,xR));
}
vector <pii> e[N];
int main(){
	n=read(),m=read();
	For(i,1,m){
		int L=read(),R=read(),c=read();
		e[L].pb(mp(R,c));
	}
	Fod(i,n-1,1){
		LL now=INF;
		for (auto p : e[i])
			ckmin(now,query(1,1,n,i+1,p.fi)+p.se);
		update(1,1,n,i,now);
	}
	LL res=query(1,1,n,1,1);
	if (res==INF)
		res=-1;
	cout<<res<<endl;
	return 0;
}

Measured memory_bytes: 17992