#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  int maxx = 0;
  int a[100009];
  map<long long int, int> M;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 1; i <= n / 2; i++)
    if (a[i] >= i) M[a[i] - i]++;
  for (int i = (n + 1) / 2 + 1; i <= n; i++)
    if (a[i] + i - (n + 1) >= 0) M[a[i] - (n + 1 - i)]++;
  if (n % 2 == 1) M[a[(n + 1) / 2] - (n + 1) / 2]++;
  for (int i = 1; i <= n / 2; i++) maxx = max(maxx, M[a[i] - i]);
  for (int i = (n + 1) / 2 + 1; i <= n; i++)
    maxx = max(maxx, M[a[i] - (n + 1 - i)]);
  if (n % 2 == 1) maxx = max(maxx, M[a[(n + 1) / 2] - 1]);
  cout << n - maxx;
}
