#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>using namespace std;const int maxn = 10010;const int maxe = 50010;struct edge {    int t;    edge* next;}e[maxe * 2], *head[maxn]; int ne = 0;int n, m;void addedge(int f, int t) {    e[ne].t = t, e[ne].next = head[f]; head[f] = e + ne ++;}void read() {    scanf("%d%d", &n, &m);    for(int i = 1; i <= m; ++ i) {        int u, v;        scanf("%d%d", &u, &v);        addedge(v, u);    }}int dfs[maxn], back[maxn];int s[maxn], top = 0, Index = 0;bool vis[maxn]; int num = 0;int size[maxn];int belong[maxn];void tarjan(int now) {    dfs[now] = back[now] = ++ Index; vis[now] = true;    s[++ top] = now;    for(edge* p = head[now]; p; p = p-> next) {        if(!dfs[p-> t]) tarjan(p-> t), back[now] = min(back[now], back[p-> t]);        else if(vis[p-> t] && back[now] > dfs[p-> t]) {             back[now] = dfs[p-> t];        }    }    if(dfs[now] == back[now]) {        ++ num;        while(1) {            belong[s[top]] = num;             size[num] ++;            vis[s[top]] = false;            if(s[top] == now) break;            top --;        }        top --;    }}int f[maxn]; int in[maxn];bool have[maxn];void sov() {    memset(vis, false, sizeof(false));    memset(size, 0, sizeof(size));    memset(have, 0, sizeof(have));    for(int i = 1; i <= n; ++ i) {        if(!dfs[i]) tarjan(i);    }    memset(in, 0, sizeof(in));    int tol = 0;    for(int i = 1; i <= n; ++ i) {        for(edge* p = head[i]; p; p = p-> next) {            if(belong[p-> t] != belong[i]) {                in[belong[p-> t]] ++;            }        }    }    int pos = 0;    for(int i = 1; i <= num; ++ i) {        if(in[i] == 0) tol ++, pos = i;    }    if(tol == 1) printf("%d\n", size[pos]);    else printf("0\n");}int main() {    //freopen("test.in", "r", stdin);    read(); sov();    return 0;}