#include <bits/stdc++.h>
#define rep(i,j,k) for(int i=(j);i<=(k);i++)
#define per(i,j,k) for(int i=(j);i>=(k);i--)
#define uint unsigned int
#define ll long long
#define ull unsigned long long
#define db double
#define ldb long double
#define pii pair<int,int>
#define pll pair<ll,ll>
#define mkp make_pair
#define eb emplace_back
#define SZ(S) (int)S.size()
//#define mod 998244353
//#define mod 1000000007
#define inf 0x3f3f3f3f
#define linf 0x3f3f3f3f3f3f3f3f
using namespace std;

template <class T>
void chkmax(T &x,T y){
	if(x < y) x = y;
}

template <class T>
void chkmin(T &x,T y){
	if(x > y) x = y;
}

inline int popcnt(int x){
	return __builtin_popcount(x);
}

inline int ctz(int x){
	return __builtin_ctz(x);
}


/*ll power(ll p,int k = mod - 2){
	ll ans = 1;
	while(k){
		if(k % 2 == 1) ans = ans * p % mod;
		p = p * p % mod;
		k /= 2;	
	}
	return ans;
}*/
int n,m;
vector <pii> S[200005];
vector <int> G[200005];
int deg[200005];
void add(int u,int v){
	G[v].eb(u);
	deg[u]++;
}
queue <int> Q;

int sz;
int ord[200005],a[200005];
int main(){	
	scanf("%d%d",&n,&m);
	rep(i,1,m){
		int x,y,z;
		scanf("%d%d%d",&x,&y,&z);
		add(x,y);add(x,z);
		S[x].eb(y,z);
	}
	rep(u,1,n) if(!deg[u]) Q.push(u);
	while(!Q.empty()){
		int u = ord[++sz] = Q.front();
		Q.pop();
		for(int v:G[u]){
			deg[v]--;
			if(!deg[v]) Q.push(v);
		}
	}
	int ssum = 0;
	rep(i,1,n){
		int u = ord[i];
		a[u] = 1;
		for(pii I:S[u]) chkmax(a[u],a[I.first] + a[I.second]);
		ssum += a[u];
		if(ssum > 1000000000){
			printf("-1\n");
			return 0;
		}
	}
	printf("%d\n",ssum);
	return 0;
}