#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<string>
#include<cstring>
#include<cstdlib>
#include<queue>

using namespace std;

inline int read()
{
    int ans=0;
    char last=‘ ‘,ch=getchar();
    while(ch<‘0‘||ch>‘9‘) last=ch,ch=getchar();
    while(ch>=‘0‘&&ch<=‘9‘) ans=ans*10+ch-‘0‘,ch=getchar();
    if(last==‘-‘) ans=-ans;
    return ans;
}

const int maxn=50010;
int n,m,edge_num=0,tim=0,top=0,qltnum=0;
int head[maxn],dfn[maxn],vis[maxn],low[maxn];
int s[maxn],color[maxn],size[maxn],out[maxn];
struct node
{
    int nxt,to;
}edge[maxn];

void addedge(int u,int v)
{
    edge_num++;
    edge[edge_num].to =v;
    edge[edge_num].nxt =head[u];
    head[u]=edge_num;
}

void tarjan(int x)
{
    dfn[x]=low[x]=++tim;
    vis[x]=1;
    s[++top]=x;
    for(int i=head[x];i;i=edge[i].nxt )
    {
        int y=edge[i].to ;
        if(!dfn[y])
        {
            tarjan(y);
            low[x]=min(low[x],low[y]);
        }
        else if(vis[y])
            low[x]=min(low[x],dfn[y]);
    }
    if(dfn[x]==low[x])
    {
        qltnum++;
        int y;
        do
        {
            y=s[top--];
            vis[y]=0;
            color[y]=qltnum;
            size[qltnum]++;
        }while(x!=y);
    }
}

int main()
{
    n=read();m=read();
    int u,v;
    for(int i=1;i<=m;i++)
    {
        u=read();v=read();
        addedge(u,v);
    }
    for(int i=1;i<=n;i++)
    {
        if(!dfn[i]) tarjan(i);
    }
    for(int i=1;i<=n;i++)
        for(int j=head[i];j;j=edge[j].nxt )
        {
            if(color[i]!=color[edge[j].to ])
            {
                out[color[i]]++;
            }
        }
    
    int now=0;
    for(int i=1;i<=qltnum;i++)
    {
        if(!out[i])
        {
            if(now)
            {
                printf("0\n");
                return 0;
            }
            now=i;
        }
    }
    
    printf("%d\n",size[now]);
    
    return 0;
}