#include <bits/stdc++.h>
using namespace std;
const int maxn = 100005;
int first[maxn];
int n;
int main() {
  int num, i;
  while (scanf("%d", &n) != EOF) {
    memset(first, 0, sizeof(first));
    for (i = 1; i <= n; i++) {
      scanf("%d", &num);
      if (i <= n / 2) {
        if (num - (i - 1) > 0) first[num - (i - 1)]++;
      } else {
        if (num - (n - i) > 0) first[num - (n - i)]++;
      }
    }
    int Max = 0;
    for (i = 1; i < maxn; i++)
      if (first[i] > Max) Max = first[i];
    printf("%d\n", n - Max);
  }
  return 0;
}
