#include <bits/stdc++.h>
using namespace std;
int a[100100], b[100100];
int main() {
  int n;
  cin >> n;
  int ans = 0;
  for (int i = 0; i < n; i++) cin >> a[i];
  for (int i = 0; i < (n + 1) / 2; i++) {
    if (a[i] - i > 0) b[a[i] - i]++;
  }
  for (int i = (n + 1) / 2; i < n; i++) {
    if (a[i] + i - n + 1 > 0) b[a[i] + i + 1 - n]++;
  }
  for (int i = 0; i <= 1e5; i++) ans = max(ans, b[i]);
  cout << n - ans << endl;
  return 0;
}
