#include <bits/stdc++.h>
using namespace std;
int A[100001], R[100001];
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", &A[i]);
  memset(R, 0, sizeof R);
  for (int i = 0; i < n; i++) {
    int p = A[i] - min(i, n - 1 - i);
    if (p >= 1) R[p]++;
  }
  int *it = max_element(R, R + 100001);
  printf("%d\n", n - *it);
  return 0;
}
