#include <bits/stdc++.h>
using namespace std;
int a[100001];
int main() {
  int n, i, j, x;
  cin >> n;
  for (i = 1; i <= n; i++) {
    j = min(i - 1, n - i);
    cin >> x;
    x = x - j;
    if (x > 0) a[x]++;
  }
  for (j = 0, i = 1; i <= 100000; i++) j = max(j, a[i]);
  cout << n - j << endl;
  return 0;
}
