#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, buf, m = -1, c = 0;
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> buf;
    if (m == -1 && buf == i) {
      m = buf;
    } else if (m != -1 && buf == i) {
      c += buf - m;
    }
  }
  cout << min(c, n - c - 1);
  return 0;
}
