#include <bits/stdc++.h>
using namespace std;
const int Maxn = 100000 + 10;
const int INF = 0x7f7f7f7f;
const double eps = 1e-8;
const double pi = 3.1415926535897932384626433832795;
int a[Maxn], cnt[Maxn];
int main() {
  int ans = 0, n, maxn = 0;
  ios::sync_with_stdio(0);
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    if (i < n / 2) {
      cnt[a[i] - i]++;
      maxn = max(maxn, a[i] - i);
    } else {
      cnt[a[i] - (n - i - 1)]++;
      maxn = max(maxn, a[i] - (n - i - 1));
    }
  }
  for (int i = 1; i <= maxn; i++) ans = max(ans, cnt[i]);
  cout << n - ans << endl;
  return 0;
}
