#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n;
  int arr[100010];
  scanf("%d", &n);
  map<int, int> mapper;
  for (int i = 0; i < n; i++) {
    scanf("%d", &arr[i]);
    if (arr[i] - min(i, n - i - 1) > 0) mapper[arr[i] - min(i, n - i - 1)]++;
  }
  int cur_min = 1000000000;
  for (map<int, int>::iterator it = mapper.begin(); it != mapper.end(); it++) {
    cur_min = min(cur_min, n - it->second);
  }
  cout << cur_min;
  return 0;
}
