#include <bits/stdc++.h>
using namespace std;
signed main() {
  std::ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vector<int> freq(2e5 + 5);
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    if (x - min(i, n - i - 1) > 0) freq[x - min(i, n - i - 1)]++;
  }
  cout << n - *max_element((freq).begin(), (freq).end()) << "\n";
}
