#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>#include <cmath>#include <string>#include <map>#include <stack>#include <vector>#include <set>#include <queue>#define inf 0x3f3f3f3f#define mod 10000typedef long long ll;using namespace std;const int N=10005;const int M=100005;int n,m,cnt,tim,top,cut;int head[N],dfn[N],low[N],stack1[N];int num[N],du[N],vis[N];struct man {    int to,next;} edg[M];void init() {    memset(dfn,0,sizeof(dfn));    memset(low,0,sizeof(low));    memset(head,-1,sizeof(head));    memset(vis,0,sizeof(vis));    memset(num,0,sizeof(num));    memset(du,0,sizeof(du));    cnt=0;    tim=1;    top=0;    cut=0;}void add(int u,int v) {    edg[cnt].to=v;    edg[cnt].next=head[u];    head[u]=cnt;    cnt++;}void dfs(int u,int fa) {    dfn[u]=tim;    low[u]=tim++;    vis[u]=1;    stack1[top++]=u;    for(int i=head[u]; i!=-1; i=edg[i].next) {        int v=edg[i].to;        if(!vis[v]) {            dfs(v,u);            low[u]=min(low[u],low[v]);        } else low[u]=min(low[u],low[v]);    }    if(low[u]==dfn[u]) {        cut++;        while(top>0&&stack1[top]!=u) {            top--;            vis[stack1[top]]=2;            num[stack1[top]]=cut;        }    }}int main() {    int u,v;    while(~scanf("%d%d",&n,&m)) {        init();        for(int i=0; i<m; i++) {            scanf("%d%d",&u,&v);            add(u,v);        }        for(int i=1; i<=n; i++) {            if(!vis[i])dfs(i,0);        }        for(int i=1; i<=n; i++) {            for(int j=head[i]; j!=-1; j=edg[j].next) {                if(num[i]!=num[edg[j].to])du[num[i]]++;            }        }        int sum=0,x;        for(int i=1; i<=cut; i++) {            if(!du[i])sum++,x=i;        }        if(sum==1) {            sum=0;            for(int i=1; i<=n; i++) {                if(num[i]==x)sum++;            }            cout<<sum<<endl;        } else puts("0");    }    return 0;}

#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>#include <cmath>#include <string>#include <map>#include <stack>#include <vector>#include <set>#include <queue>#define inf 0x3f3f3f3f#define mod 10000typedef long long ll;using namespace std;const int N=10005;const int M=100005;struct Node{    int to,next;}edge1[N*5],edge2[N*5];//edge1用来存原图G，edge2用来存GT，即逆图//都是用邻接表来储存的图，int head1[N];//原图的邻接表的头结点int head2[N];//逆图的邻接表的头结点int mark1[N],mark2[N];int tot1,tot2;int cnt1,cnt2,st[N],belong[N];int num,setNum[N];struct Edge{    int l,r;}e[N*5];//储存边void add(int a,int b)//添加边a->b，在原图和逆图都需要添加{    edge1[tot1].to=b;edge1[tot1].next=head1[a];head1[a]=tot1++;//邻接表存的原图    edge2[tot2].to=a;edge2[tot2].next=head2[b];head2[b]=tot2++;//邻接表存的逆图}void DFS1(int x)//深度搜索原图{    mark1[x]=1;    for(int i=head1[x];i!=-1;i=edge1[i].next)          if(!mark1[edge1[i].to])  DFS1(edge1[i].to);    st[cnt1++]=x;//st数组是按照完成时间从小到大排序的}void DFS2(int x)//深度搜索逆图{    mark2[x]=1;    num++;    belong[x]=cnt2;    for(int i=head2[x];i!=-1;i=edge2[i].next)       if(!mark2[edge2[i].to]) DFS2(edge2[i].to);}int main(){    int n,m;    while(scanf("%d%d",&n,&m)!=EOF)    {        tot1=tot2=1;        for(int i=1;i<=n;i++)//初始化        {            head1[i]=head2[i]=-1;            mark1[i]=mark2[i]=0;        }        for(int i=1;i<=m;i++)        {            int w,v;            scanf("%d%d",&w,&v);            e[i].l=w;e[i].r=v;//储存边            add(w,v);//建立邻接表        }        cnt1=cnt2=1;        for(int i=1;i<=n;i++)        {            if(!mark1[i])DFS1(i);        }        for(int i=cnt1-1;i>=1;i--)        {            if(!mark2[st[i]])            {                num=0;                DFS2(st[i]);                setNum[cnt2++]=num;            }        }        int de[N];//计算出度        memset(de,0,sizeof(de));        for(int i=1;i<=m;i++)//计算各个DAG图的出度        {            if(belong[e[i].l]!=belong[e[i].r])//原图的边不属于同一连通分支                de[belong[e[i].l]]++;        }        //计算DAG出度为0的个数        int cnt=0,res;        for(int i=1;i<cnt2;i++)            if(!de[i]){cnt++;res=i;}        if(cnt>1)  printf("0\n");        else  printf("%d\n",setNum[res]);    }    return 0;}