#include <bits/stdc++.h>
using namespace std;
long n, q, bd, mx;
map<long, long> calc;
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n;
  for (long i = 1; i <= n; i++) {
    cin >> q;
    bd = min(i - 1, n - i);
    bd = q - bd;
    if (bd <= 0) continue;
    mx = max(mx, calc[bd] + 1);
    calc[bd]++;
  }
  cout << n - mx << endl;
  cin.get();
  cin.get();
  return 0;
}
