/*

  tarjan缩点后，统计每个所点的出度，当有且只有一个出度为0的缩点是，

  答案即为这个缩点所包含的点的数量，否则答案为0

*/

#include<cstdio>

#include<iostream>

#include<cstring>

#include<vector>

#include<stack>

#define M 10010

using namespace std;

int low[M],num[M],belong[M],vis[M],instack[M],in[M],out[M],sum[M],indexx,cnt,n,m;

vector<int> grap[M];

stack<int> s;

void tarjan(int v)

{

    low[v]=num[v]=++indexx;

    s.push(v);

    vis[v]=;

    instack[v]=;

    for(int i=;i<grap[v].size();i++)

    {

        int w=grap[v][i];

        if(!vis[w])

        {

            tarjan(w);

            low[v]=min(low[v],low[w]);

        }

        else if(instack[w])

          low[v]=min(low[v],num[w]);

    }

    int u;

    if(low[v]==num[v])

    {

        ++cnt;

        do

        {

            u=s.top();

            s.pop();

            belong[u]=cnt;

            sum[cnt]++;

            instack[u]=;

        }

        while(u!=v);

    }

}

void work()

{

    for(int i=;i<=m;i++)

    {

        int x,y;

        scanf("%d%d",&x,&y);

        grap[y].push_back(x);

    }

    for(int i=;i<=n;i++)

      if(!vis[i])

        tarjan(i);

    for(int i=;i<=n;i++)

      for(int j=;j<grap[i].size();j++)

        if(belong[i]!=belong[grap[i][j]])

        {

            in[belong[i]]++;

            out[belong[grap[i][j]]]++;

        }

    int tot=,p;

    for(int i=;i<=cnt;i++)

      if(!out[i])

      {

          tot++;

          p=i;

      }

    if(tot==)printf("%d\n",sum[p]);

    else printf("0\n");

}

int main()

{

    while(scanf("%d%d",&n,&m)==)

    {

        work();

        memset(low,,sizeof(low));

        memset(num,,sizeof(num));

        memset(belong,,sizeof(belong));

        memset(vis,,sizeof(vis));

        memset(instack,,sizeof(instack));

        memset(in,,sizeof(in));

        memset(out,,sizeof(out));

        memset(sum,,sizeof(sum));

        for(int i=;i<=n;i++)grap[i].clear();

        while(!s.empty())s.pop();

        indexx=;

        cnt=;

    }

}