#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using pll = pair<ll, ll>;
using pii = pair<int, int>;
const int MOD = 1e9 + 7;
const int MAX = 1e5 + 5;
const ll INF = 1e18 + 1;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  vector<int> v(n), k(n);
  map<int, int> mp;
  for (int i = 0; i < n; i++) {
    cin >> v[i];
    k[i] = v[i] - min(i + 1, n - i);
    if (k[i] >= 0) mp[k[i]]++;
  }
  int mx = 0;
  for (auto i : mp) mx = max(mx, i.second);
  cout << n - mx;
  return 0;
}
