#include<bits/stdc++.h>				//Written by ThiDaiLoc
using namespace std;				//Team Three Wolves
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll,ll> pll;
typedef pair<ll,pll> trp;
typedef vector<ll> vi;
typedef vector<pll> vp;

#define fu(i,a,b)   for(ll i=a;i<=b;i++)
#define f1(i,n)     for(ll i=1;i<=n;i++)
#define fs(i,s)     for(ll i=0;i+1<=s.length();i++)
#define fd(i,b,a)   for(ll i=b;i>=a;i--)
#define fuv(i,a)    for(ll i=0;i<a.size();i++)
#define fdv(i,a)    for(ll i=(ll)a.size()-1;i>=0;i--)
#define ms(a,x)     memset(a, x, sizeof a)
#define prec(n)     fixed<<setprecision(n)
#define uni(a)      (a).erase(unique(all(a)), (a).end())
#define pb(i)       push_back(i)
#define popb()      pop_back()
#define sc(a)       cin>>a
#define sc2(a,b)    cin>>a>>b
#define pr(a)       cout<<a<<endl
#define pr2(a,b)    cout<<a<<" "<<b<<endl
#define rpr(a)      return cout<<a<<endl,0
#define prY         cout<<"YES"<<endl
#define prN         cout<<"NO"<<endl
#define bit(n,i)    (((n)>>(i))&1)
#define lowb(a,n,x) lower_bound(a,a+n,x) -a
#define lowb2(a,x)  lower_bound(all(a),x) -a.begin()
#define all(x)      (x).begin(), (x).end()
#define sz(a)       (ll)a.size()
#define le(s)       (ll)s.length()		
#define ast(x,a,b)  assert(x>=a and x<=b)				
#define re          return 	
#define	mp(a,b)     make_pair(a,b)
#define	mp3(a,b,c)  make_pair(a,make_pair(b,c))
#define se          second
#define fi          first 
#define sse         second.second
#define sfi         second.first
#define debug(x)    cerr << #x << " = " << x << endl
#define INPUT       freopen("locin.txt", "r", stdin)
#define OUTPUT      freopen("locout.txt", "w", stdout)
inline ll isqrt(ll k) {ll r = sqrt(k) + 1; while (r * r > k) r--; return r;}
inline ll icbrt(ll k) {ll r = cbrt(k) + 1; while (r * r * r > k) r--; return r;}
inline ll mnz(ll& a,ll b){return a=(a>b?b:a);}
inline ll mxz(ll& a,ll b){return a=(a<b?b:a);}
inline string toString(ll n) {stringstream ss; ss << n;return ss.str();}
double const eps = 1e-6;
ll const Base=998244353,oo=1e17,MAXN=1e5;


class node{
	public :ll val,l,r;
};
node segtree[4*MAXN+5];
void Init(ll x,ll l,ll r){
	segtree[x].val=oo;
	segtree[x].l=l;
	segtree[x].r=r;
	
	if(l==r)re;
	ll mid=(l+r)>>1LL;
	Init(x<<1,l,mid);
	Init((x<<1)|1,mid+1,r);
}

void inline Upd(ll x,ll u,ll v,ll val){
	if(u>segtree[x].r or v<segtree[x].l)re;
	if(u<=segtree[x].l and v>=segtree[x].r){
		mnz(segtree[x].val,val);
		re ;
	}
	Upd(x<<1,u,v,val);
	Upd((x<<1)|1,u,v,val);
}
ll inline Get(ll x,ll p){
	if(segtree[x].l>p or segtree[x].r<p)re oo;
	if(segtree[x].r==segtree[x].l)re segtree[x].val;
	re min(segtree[x].val,min(Get(x<<1,p),Get(((x<<1)|1),p)));
}
vp store[MAXN+5];
ll Solves(){
	
	ll n,m,k,cnt=0,ans=1,x,y,q,c,sum=0,v,t;
	// PROCESS IN HERE
	
	sc(n);
	sc(m);
	Init(1,1,n);
	Upd(1,1,1,0);
	while(m--){
		sc2(x,y); sc(c);
		store[x].pb(mp(y,c));
	}
	
	f1(i,n){
		v=Get(1,i);	
		fuv(j,store[i]){
			Upd(1,i,store[i][j].fi,v+store[i][j].se);
		}
	}
	c=Get(1,n);
	if(c==oo)c=-1;
	pr(c);
	re 0;
	// Hack it if you can :)
}

int main(){
	//INPUT;
	if(fopen("locin.txt", "r"))INPUT;
	ll JUDGE_ONLINE = 1;
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	ll test=1;
//	sc(test);
	fu(T,1,test){
//		cout<<"Case #"<<T<<": ";
		Solves();
	}
	if (!JUDGE_ONLINE) cout << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";  
		
}

Measured memory_bytes: 13568