#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];
int d[N]; int in[N];
signed main() {
	NOSYNC;
	cin >> n >> m;
	for(int i = 1; i <= m; i ++) {
		int x, y, z;
		cin >> x >> y >> z;
		g[y].push_back(x);
		g[z].push_back(x);
		d[x] += 2;	
	}
	queue<int> q;
	for(int i = 1; i <= n; i ++) {
		if(!d[i]) {
			q.push(i);
			in[i] = 1;
			a[i] = 1;
		}
	}
	while(!q.empty()) {
		int u = q.front();
		q.pop();
		for(int v : g[u]) {
			-- d[v];
			a[v] += a[u];
			if(!d[v]) {
				q.push(v);
				in[v] = 1;
			}
		}
	}
	int ans = 0;
	for(int i = 1; i <= n; i ++) {
		if(!in[i]) {
			cout << -1 << endl;
			return 0;
		}
		ans += a[i];
	}
	cout << ans << endl;
	
	return 0;
}