#include <bits/stdc++.h>
using namespace std;
#define MAXN 100001
#define MAXM 5*MAXN

int n, m, u, v, cnt, ssum, ans;
int dfn[MAXN], low[MAXN], scc[MAXN], siz[MAXN];
int us[MAXM], vs[MAXM];
vector<int> e[MAXN];
bitset<MAXN> ins, in;
stack<int> st;
queue<int> que;

void tarjan(int x){
	low[x] = dfn[x] = ++cnt;
	st.push(x);
	ins.set(x);
	for (auto i: e[x]){
		if (!dfn[i]){
			tarjan(i);
			low[x] = min(low[x], low[i]);
		}else if (ins.test(i)){
			low[x] = min(low[x], dfn[i]);
		}
	}
	if (dfn[x] == low[x]){
		scc[x] = ++ssum;
		++siz[ssum];
		while (st.top() != x){
			scc[st.top()] = ssum;
			ins.reset(st.top());
			st.pop();
			++siz[ssum];
		}
		st.pop();
		ins.reset(x);
	}
}

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	
	cin >> n >> m;
	for (int i(1); i<=m; ++i){
		cin >> u >> v;
		e[u].push_back(v);
		us[i] = u;
		vs[i] = v;
	}
	for (int i(1); i<=n; ++i){
		if (!dfn[i]) tarjan(i);
	}
	
	for (int i(1); i<=m; ++i){
		u = scc[us[i]];
		v = scc[vs[i]];
		if (u != v) in.set(v); 
	}
	
	for (int i(1); i<=ssum; ++i){
		if (!in.test(i)){
			if (ans){
				cout << 0;
				return 0;
			}
			ans = i;
		}
	}
	cout << siz[ans];
	
	return 0;
}

感觉和题解亿模亿样，找不到错误。