#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) cin >> arr[i];
  map<int, int> mp;
  for (int i = 0; i < (n / 2); i++)
    if (arr[i] - i > 0) mp[arr[i] - i]++;
  for (int i = 0; i < (n / 2); i++)
    if (arr[n - i - 1] - i > 0) mp[arr[n - i - 1] - i]++;
  if (n % 2 != 0)
    if (arr[n / 2] - n / 2 > 0) mp[arr[n / 2] - n / 2]++;
  int mx = 0, start;
  for (auto x : mp)
    if (x.second > mx) mx = x.second, start = x.first;
  int ans = 0;
  for (int i = 0; i < (n / 2); i++) {
    if (arr[i] != start) ans++;
    if (arr[n - i - 1] != start) ans++;
    start++;
  }
  if (n % 2 != 0 && arr[n / 2] != start) ans++;
  cout << ans;
  return 0;
}
