#include <bits/stdc++.h>
using namespace std;
int a[200000];
int main() {
  int i, j, k, l, n, m;
  map<int, int> M;
  while (cin >> n) {
    for (i = 0; i < n; i++) cin >> a[i];
    M.clear();
    m = 0;
    for (i = 0; i < n; i++) {
      l = a[i] - min(i, n - 1 - i);
      M[l]++;
      if (l > 0) m = max(m, M[l]);
    }
    cout << (n - m) << endl;
  }
  return 0;
}
