#include<iostream>
#include<stdio.h>
#include<vector>
#include<cstring>
using namespace std;
#define maxn 10005
vector<int> E[maxn];
vector<int> rE[maxn];
int belong[maxn];
int vis[maxn];
int cl[maxn];
int cnt = 0;
int siz[maxn];
int kis[maxn];
void dfs1(int x)
{
    vis[x]=1;
    for(int i=0;i<E[x].size();i++)
    {
        int e = E[x][i];
        if(vis[e])continue;
        dfs1(e);
    }
    cl[++cnt]=x;
}
void dfs2(int x)
{
    vis[x]=1;
    belong[x]=cnt;
    siz[cnt]++;
    for(int i=0;i<rE[x].size();i++)
    {
        int e = rE[x][i];
        if(vis[e])continue;
        dfs2(e);
    }
}
int main()
{
    int n,m;scanf("%d%d",&n,&m);
    for(int i=0;i<m;i++)
    {
        int x,y;scanf("%d%d",&x,&y);
        E[x].push_back(y);
        rE[y].push_back(x);
    }
    for(int i=1;i<=n;i++)
    {
        if(!vis[i])
            dfs1(i);
    }
    memset(vis,0,sizeof(vis));
    cnt = 0;
    for(int i=n;i>=1;i--)
    {
        if(!vis[cl[i]])
        {
            cnt++;
            dfs2(cl[i]);
        }
    }
    for(int i=1;i<=n;i++)
    {
        for(int j=0;j<E[i].size();j++)
        {
            if(belong[i]==belong[E[i][j]])
                continue;
            kis[belong[i]]++;
        }
    }
    int ans1=0,ans2=0;
    for(int i=1;i<=cnt;i++)
    {
        if(kis[i]==0)
        {
            ans1++;
            ans2=i;
        }
    }
    if(ans1==1)
        printf("%d\n",siz[ans2]);
    else
        puts("0");
}