#include <bits/stdc++.h>
using namespace std;
const int N = 1e4 + 10, M = 5e4 + 10;
int n, m;

int h[N], e[M], ne[M], idx;
int dnf[N], low[N], timestamp;//时间戳
int stk[N], top;//栈
bool in_stk[N];//是否在栈中
int id[N], scc_cnt;
int psize[N];
int aout[N];

void add(int a, int b)
{
    e[idx] = b, ne[idx] = h[a], h[a] = idx ++;
}

void tarjin(int u)
{
    dnf[u] = low[u] = ++ timestamp;
    stk[++ top] = u, in_stk[u] = true;

    for (int i = h[u]; ~i; i = ne[i])
    {
        int j = e[i];
        if (!dnf[j])
        {
            tarjin(j);
            low[u] = min(low[u], low[j]);
        }
        else if (in_stk[j])
            low[u] = min(low[u], dnf[j]);
    }

    if (dnf[u] == low[u])
    {
        ++ scc_cnt;
        int y;
        do{
            y = stk[top --];
            in_stk[y] = false;
            id[y] = scc_cnt;
            psize[scc_cnt] ++;
        } while(y != u);
    }
}

int main(void)
{
    cin >> n >> m;
    memset(h, -1, sizeof h);
    for (int i = 0; i < m; i ++)
    {
        int a, b;
        cin >> a >>b;
        add(a, b);
    }

    for (int i = 1; i <= n; i ++)
        if (!dnf[i])
            tarjin(i);

    for (int i = 1; i <= n; i ++)
        for (int j = h[i]; ~j; j = ne[j])
        {
            int k = e[j];
            int a = id[i], b = id[k];
            if (a != b)
            {
                aout[a] ++;
            }
        }
    int sizep = 0, sum = 0;
    for (int i = 1; i <= scc_cnt; i ++)
        if (!aout[i])
        {
            sizep ++;
            sum += psize[i];
            if (sizep > 1)
            {
                sum = 0;
                break;
            }
        }
    printf("%d\n", sum);
    return 0;
}