#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n];
  int cnt = 0;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    if (a[i] != i + 1) cnt++;
  }
  int ans = (n - cnt) / 2;
  if ((n - cnt) % 2 == 1) ans++;
  cout << ans << endl;
  return 0;
}
