#include <bits/stdc++.h>
#define NOSYNC ios::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define all(x) x.begin(), x.end()
#define endl '\n'
using namespace std;
using ll = long long;
using ull = unsigned long long;

const int N = 200005;

// -------------- Templates Above -------------------------

int a[N]; int n, m;
vector<int> G[N]; vector<int> g[N];
int in[N]; int vis[N]; int d[N];
vector<int> ano[N]; int vq[N];

signed main() {
	NOSYNC;
	cin >> n >> m;
	for(int i = 1; i <= m; i ++) {
		int x, y, z;
		cin >> x >> y >> z;	
		G[x].push_back(y);
		G[x].push_back(z);
		++ d[y]; ++ d[z];
		ano[y].push_back(z);
		ano[z].push_back(y);
		g[y].push_back(x);
		g[z].push_back(x);
		in[x] += 2;
	}
	
	queue<int> q;
	for(int i = 1; i <= n; i ++) {
		if(!d[i]) {
			q.push(i);
		}
	}
	
	while(!q.empty()) {
		int u = q.front(); q.pop();
		vis[u] = 1;
		for(int v : G[u]) {
			-- d[v];
			if(!d[v]) {
				q.push(v);
			}
		}
	}
	for(int i = 1; i <= n; i ++) {
		if(!vis[i]) {
			cout << -1 << endl;
			return 0;
		}
	}
	
	for(int i = 1; i <= n; i ++) vis[i] = 0;
	
	queue<int> q1;
	
	for(int i = 1; i <= n; i ++) {
		if(!in[i]) {
			q1.push(i);
			a[i] = 1;
		}
	}
	while(!q1.empty()) {
		int u = q1.front(); q1.pop();
		vq[u] = 1;
		for(int v : g[u]) {
			for(int w : ano[u]) {
				if(!vq[w]) continue;
				// cout << "u: " << u << " v: " << v << " w: " << w << endl;
				a[v] = max(a[v], a[u] + a[w]);
			}
			-- in[v];
			if(!in[v]) {
				q1.push(v);
			}
		}
	}
	
	
	
	int ans = 0;
	for(int i = 1; i <= n; i ++) {
		ans += a[i];
	}
	if(ans > 1e9 ) {
		cout << -1 << endl;
	}
	cout << ans << endl;
	return 0;
}