#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a(n), b(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i] >> b[i];
  }
  int res = 0;
  for (int i = 0; i < n; i++) {
    bool can = false;
    for (int j = 0; j < n; j++) {
      if (i == j) continue;
      if (b[j] == a[i]) can = true;
    }
    res += (!can);
  }
  cout << res;
}
