#include <bits/stdc++.h>
using namespace std;
const int MAX = 100015;
int n, a[MAX], m;
int main() {
  cin >> n;
  int mid = (n - 1) / 2;
  for (int i = 0; i < n; i++) {
    scanf("%d", &m);
    if (i <= mid && m > i)
      a[m - i]++;
    else if (i > mid && m > (n - i - 1))
      a[m - (n - i - 1)]++;
  }
  int ret = 0;
  for (int i = 0; i < MAX; i++) ret = max(ret, a[i]);
  cout << n - ret;
  return 0;
}
