3 3
1 2
2 1
2 3

 1

#include<bits/stdc++.h>
using namespace std;
const int N=5e4+10;
int n,m,x,y;
int sta[N],head[N],dfn[N],low[N];
int top,tot,Time,cnt,g[N],sum[N];
bool vis[N];
int cd[N];
struct edge{
    int to;
    int ne;
}e[N];

void add(int u,int v){
    e[++cnt].to = v;
    e[cnt].ne = head[u];
    head[u] = cnt;
}

void tarjan(int u){//缩点板子基操
    dfn[u]=low[u]=++Time;
    vis[u]=1;sta[++top]=u;
    for(int i=head[u];i;i=e[i].ne){
        int v = e[i].to;
        if(!dfn[v]){
            tarjan(v);
            low[u] = min( low[u] , low[v]);
        }
        if(vis[v]){
            low[u] = min(low[u] , dfn[v]);
        }
    }
    if(dfn[u] == low[u]){
        tot++;
        while(sta[top+1]!=u){
            int v = sta[top--];
            vis[v] = 0;
            g[v] = tot;
            sum[tot]++;
        }
    }
}

int main(){
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++){
        scanf("%d%d",&x,&y);
        add(x,y);
    }
    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=e[j].ne){
            int v = e[j].to;
            if(g[i]!=g[v]){
                cd[g[i]]++;
            }
        }
    }
    int pos=0;
    for(int i=1;i<=tot;i++){
        if(!cd[i]){
            if(pos){//如果有第二个出度为零的点就结束
                printf("0\n");
                return 0;
            }
            pos=i;
        }
    }
    printf("%d\n",sum[pos]);
    return 0;
}