#include <bits/stdc++.h>
using namespace std;
int arr[100001];
int cnt[100001];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL), cout.tie(NULL);
  int n;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> arr[i];
    int idx = min(i, n - i - 1);
    if (arr[i] - idx > 0) {
      ++cnt[arr[i] - idx];
    }
  }
  int mx = 0;
  for (int i = 0; i <= 100000; ++i) {
    mx = max(mx, cnt[i]);
  }
  cout << n - mx << '\n';
  return 0;
}
