#include <bits/stdc++.h>
#define ll long long
using namespace std;

vector<int>g[200005];
queue<int>q;
int a[200005]={0};
int b[200005]={0};
int d[200005]={0};
ll res=0;

void dfs(int u,int v) {
	if (d[u]<v) {
		res+=v-d[u];
		d[u]=v;
		for (auto x:g[u]) {
			dfs(x,v+1);
		}
	}
}

int main()
{
	int n,m;
	cin>>n>>m;
	int x,y,z;
	while (m--) {
		cin>>x>>y>>z;
		a[x]+=2,b[x]+=2;
		g[y].push_back(x);
		g[z].push_back(x);
	}
	for (int i=1;i<=n;i++) {
		if (a[i]==0) {
			q.push(i);
		}
	}
	while (!q.empty()) {
		for (auto x:g[q.front()]) {
			a[x]--;
			if (a[x]==0) {
				q.push(x);
			}
		}
		q.pop();
	}
	for (int i=1;i<=n;i++) {
		if (a[i]) {
			cout<<-1<<endl;
			return 0;
		}
	}
	for (int i=1;i<=n;i++) {
		if (b[i]==0 && d[i]==0) {
			dfs(i,1);
		}
		//cout<<d[i]<<endl;
	}
	cout<<res<<endl;
	return 0;
}