#define _CRT_SECURE_NO_WARNINGS
#define _SILENCE_CXX17_C_HEADER_DEPRECATION_WARNING
#include <bits/stdc++.h>
using namespace std;
const int N = 1050;
bool vis[N];
int nxt[N], n;
void dfs(int x)
{
    for (int i = 1; i <= n; i++)
        if (nxt[i] != i && !vis[nxt[i]])
        {
            vis[nxt[i]] = true;
            dfs(nxt[i]);
        }
}
int main()
{
    scanf("%d", &n);
    for (int i = 1; i <= n; i++)
    {
        int a, b;
        scanf("%d%d", &a, &b);
        nxt[a] = b;
    }
    int ans = n;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            if (nxt[i] == j && !vis[j] && j != i)
                vis[j] = true, ans--;
    //for (int i = 1; i <= n; i++) dfs(i);
    //printf("%d", accumulate(vis + 1, vis + n + 1, 0, [](auto a, auto b) { return a + !b; }));
    printf("%d", ans);
    return 0;
}