#include <iostream>#include <cstdlib>#include <cstring>#include <cstdio>using namespace std;#define maxn 10005#define maxm 50005struct Edge{ int v, next;} edge[maxm], opedge[maxm];int n, m, head[maxn], ophead[maxn], ncount, nowtime, pos[maxn *2], sig[maxn], signnum, in[maxn];int pointnum[maxn];bool flag[maxn];void addedge(int a, int b){ edge[ncount].v = b; edge[ncount].next = head[a]; head[a] = ncount; opedge[ncount].v = a; opedge[ncount].next = ophead[b]; ophead[b] = ncount; ncount++;}void input(){ scanf("%d%d", &n, &m); for (int i =0; i < m; i++) { int a, b; scanf("%d%d", &a, &b); a--; b--; addedge(a, b); }}void dfs(int a){ flag[a] =true; pos[nowtime] = a; nowtime++; for (int i = head[a]; i !=-1; i = edge[i].next) if (!flag[edge[i].v]) dfs(edge[i].v); pos[nowtime] = a; nowtime++;}void rdfs(int a){ flag[a] =true; sig[a] = signnum; pointnum[sig[a]]++; for (int i = ophead[a]; i !=-1; i = opedge[i].next) if (!flag[opedge[i].v]) rdfs(opedge[i].v);}int main(){ ncount =0; memset(head, -1, sizeof(head)); memset(ophead, -1, sizeof(ophead)); memset(flag, 0, sizeof(flag)); input(); nowtime =1; for (int i =0; i < n; i++) if (!flag[i]) dfs(i); memset(flag, 0, sizeof(flag)); memset(pointnum, 0, sizeof(pointnum)); signnum =0; for (int i =2* n; i >0; i--) if (!flag[pos[i]]) { rdfs(pos[i]); signnum++; } memset(in, 0, sizeof(in)); ncount =0; for (int i =0; i < n; i++) for (int j = head[i]; j !=-1; j = edge[j].next) if (sig[i] != sig[edge[j].v]) in[sig[i]]++; int ans =-1; bool ok =true; for (int i =0; i < signnum; i++) if (in[i] ==0) { if (ans !=-1) { ok =false; break; } ans = i; } if (ok) printf("%dn", pointnum[ans]); else printf("0n"); return 0;}
