#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC target("sse4")
using namespace std;
const int maxn = 2e5 + 10;
const int mod = 1000000007;
const long long INF = 1e18;
const int MX = 2000000001;
long long t[maxn], a;
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0);
  long long tr, q, n, ar, b, k, l, mn, r, x = 0, y = 0, ans = 0, temp = 0,
                                          res = 0;
  cin >> n;
  for (long long i = 1; i <= n; i++) {
    cin >> a;
    a -= min(i, n - i + 1);
    if (a >= 0) t[a]++, ans = max(ans, t[a]);
  }
  cout << n - ans;
}
