RT

#include <iostream>
#include <cstring>
#include <vector>
#include <cmath>
#include <algorithm>
#include <climits>
#include <queue>
#include <map>
#include <stack>

#define endl "\n"
#define IOS ios::sync_with_stdio(false), cin.tie(0), cout.tie(0)
#define int long long
#define ULL unsigned long long
#define ls(k) k << 1
#define rs(k) k << 1 | 1
#define mid(a, b) (a + b) >> 1 

const int N = 2e5 + 6, M = 520;

#define zhengdongwen using 
#define AK namespace 
#define IOI std

zhengdongwen AK IOI;

inline int read(){
    int num = 0;
    char c;
    bool flag = false;
    while((c = getchar()) == ' ' || c == '\n' || c == '\r');
    if(c == '-') flag = true;
    else num = c - '0';
    while(isdigit(c = getchar())) num = num * 10 + c - '0';
    return (flag ? -1 : 1) * num;
} 

int n, m;

int h[N], edges, num[N], low[N], dfn, res;

int degree[N], idx[N], cnt[N];

stack <int> st;

bool insta[N];

struct edge

{
	int nx, to; 
}e[N];

//出度
//每个点强联通分量的编号
//每个强联通分量中点的个数 

void add(int a, int b)

{
	edges ++;
	e[edges].nx = h[a], e[edges].to = b, h[a] = edges;
}

void tarjan(int u)

{
	num[u] = low[u] = ++ dfn;
	st.push(u);
	insta[u] = true;
	for (int i = h[u]; i; i = e[i].nx)
	{
		int j = e[i].to;
		if (!num[j])
		{
			tarjan(j);
			low[u] = min(low[u], low[j]);
		}
		else if (insta[j]) low[u] = min(low[u], num[j]);
	}
	int k;
	while (low[u] == num[u])
	{
		++ res;
		do
		{
			k = st.top(), st.pop();
			insta[k] = false;
			idx[k] = res, cnt[res] ++;//编号 个数 
		}while (u != k);
	}
}

signed main()

{
	IOS;
	
	n = read(), m = read();
	while (m -- ) 
	{
		int x = read(), y = read();
		add(x, y);
	}
	for (int i = 1; i <= n; i ++ )
	{
		if (!num[i]) tarjan(i);
	}
	//system("pause");
	for (int i = 1; i <= n; i ++ )
	{
		for (int j = h[i]; j; j = e[j].nx)
		{
			int to = e[j].to;
			if (idx[i] != idx[to]) degree[idx[i]] ++;//记录出度 
		}
	}
	int jud = 0;
	for (int i = 1; i <= res; i ++ )
	{
		if (!degree[i])
		{
			if (jud) 
			{
				cout << "0" << endl;
				return 0;
			}
			else jud = i;
		}
	}
	cout << cnt[jud];
}