#include <bits/stdc++.h>
using namespace std;
const int maxN = 1 << 17;
int cnt[maxN];
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    a[i] -= min(i, n - 1 - i);
    cnt[max(a[i], 0)]++;
  }
  int ans = n;
  for (int i = 1; i < maxN; i++) {
    ans = min(ans, n - cnt[i]);
  }
  cout << ans << endl;
  return 0;
}
