#include <cstdio>
inline int min(int a, int b) { return a < b ? a : b; }
int head[10010], next[50010], to[50010], ecnt;
int dfn[10010], low[10010], stk[10010], scc[10010], top, idx, scccnt;
bool instk[10010];
int deg[10010];
inline void addEdge(int f, int t)
{
    ecnt++;
    next[ecnt] = head[f];
    head[f] = ecnt;
    to[ecnt] = t;
}
void tarjan(int x)
{
    dfn[x] = low[x] = ++idx;
    instk[stk[top++] = x] = true;
    for (int cur = head[x]; cur; cur = next[cur])
        if (!dfn[to[cur]]) tarjan(to[cur]), low[x] = min(low[x], low[to[cur]]);
        else if (instk[to[cur]])
            low[x] = min(low[x], dfn[to[cur]]);
    if (dfn[x] == low[x])
    {
        scccnt++;
        do
        {
            top--;
            scc[stk[top]] = scccnt;
            instk[stk[top]] = false;
        } while (stk[top] != x);
    }
}
int main()
{
    int n, m;
    scanf("%d%d", &n, &m);
    for (int i = 0, x, y; i < m; i++)
    {
        scanf("%d%d", &x, &y);
        addEdge(x, y);
    }
    for (int i = 1; i <= n; i++)
        if (!dfn[i])
            tarjan(i);
    for (int i = 1; i <= n; i++)
        for (int cur = head[i]; cur; cur = next[cur])
            if (scc[i] != scc[to[cur]])
                deg[scc[i]]++;
    int zcnt = 0, id = 0;
    for (int i = 1; i <= scccnt; i++)
        if (deg[i] == 0)
            zcnt++, id = i;
    if (zcnt != 1) putchar('0');
    else
    {
        int ans = 0;
        for (int i = 1; i <= n; i++)
            if (scc[i] == id)
                ans++;
        printf("%d", ans);
    }
    return 0;
}