#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> v(n);
  for (int i = 0; i < n; ++i) cin >> v[i];
  vector<int> v2 = v;
  int k = n / 2;
  for (int i = 0; i < k; ++i) v[i] -= i;
  if (n % 2) v[k] -= k;
  for (int i = 0; i < k; ++i) v[n - 1 - i] -= i;
  map<int, int> mm;
  for (int i = 0; i < (int)(v).size(); ++i) {
    bool ok = true;
    if (i < k && v2[i] <= i) ok = false;
    if (i >= k && v2[i] <= n - i - 1) ok = false;
    if (ok) mm[v[i]]++;
  }
  int aux = 0;
  for (__typeof((mm).begin()) i = (mm).begin(); i != (mm).end(); ++i)
    aux = max(aux, i->second);
  cout << n - aux << endl;
}
