#include <bits/stdc++.h>
using namespace std;
void Yahia74() {}
const int N = 1e5 + 74, M = 1e5 + 74, OO = 0x3f3f3f3f;
int main() {
  Yahia74();
  int n, cnt[N] = {};
  cin >> n;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    if (i < n / 2) {
      if (x - i >= 0) cnt[x - i]++;
    } else {
      if (x - (n - i - 1) >= 0) cnt[x - (n - i - 1)]++;
    }
  }
  int mx = 0;
  for (int i = 1; i < 100001; i++) mx = max(mx, cnt[i]);
  cout << n - mx << endl;
  return 0;
}
