#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n], b[1000000] = {0};
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (int i = 0; i < n / 2; i++) {
    if (a[i] - i - 1 >= 0) b[a[i] - i - 1]++;
    if (a[n - 1 - i] - i - 1 >= 0) b[a[n - 1 - i] - i - 1]++;
  }
  if (n % 2 && a[n / 2] - (n / 2 + 1) >= 0) {
    b[a[n / 2] - (n / 2 + 1)]++;
  }
  int m = -1;
  for (int i = 0; i < 1000000; i++) m = m > b[i] ? m : b[i];
  cout << n - m;
  return 0;
}
