#include <bits/stdc++.h>
int main() {
  int n;
  std::cin >> n;
  std::vector<int> a(n);
  for (int i = 0; i < n; ++i) {
    std::cin >> a[i];
  }
  std::vector<int> good(100001, 0);
  for (int i = 0; i < n; ++i) {
    int cur = a[i] - std::min(i, n - 1 - i);
    if (cur > 0) {
      good[cur]++;
    }
  }
  int best = 0;
  for (int i = 1; i <= 100000; ++i) {
    if (good[i] > best) {
      best = good[i];
    }
  }
  int res = n - best;
  std::cout << res << "\n";
  return 0;
}
