#include <bits/stdc++.h>
using namespace std;
int n;
int a[100005];
int main() {
  std::ios::sync_with_stdio(false);
  cin >> n;
  int m;
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    cin >> m;
    int num = min(i, n - i + 1);
    m -= num;
    if (m >= 0) {
      a[m]++;
      ans = max(ans, a[m]);
    }
  }
  cout << n - ans << endl;
}
