#include <bits/stdc++.h>
using namespace std;
const int M = 5e6 + 5;
const long long int mod = 1e9 + 7;
const long long int inf = 9e18;
const double pi = 3.14159265358979323846;
long long int t, n, m, x, sum, k, p, u = 0;
string s, z, ans = "";
long long int P[10000005] = {0};
long long int raised(long long int x, long long int v) {
  if (v == 0) return 1 % mod;
  if (v % 2 == 1) return (x * raised(x, v - 1)) % mod;
  long long int r = raised(x, v / 2);
  return (r * r) % mod;
}
bool prime(long long int n) {
  if (n == 1) return 1;
  for (int i = 2; i <= sqrt(n); i++)
    if (n % i == 0) return 0;
  return 1;
}
long long int hcf(long long int a, long long int b) {
  if (a % b == 0) return b;
  return hcf(b, a % b);
}
long long int a, b;
bool check(string a, string b) {
  int n = a.size();
  int m = b.size();
  int c = 0;
  for (int i = 1; i <= min(n, m); i++) {
    if (a[n - i] != b[m - i]) {
      return 0;
    }
    if (a[n - i] == 'a' || a[n - i] == 'e' || a[n - i] == 'i' ||
        a[n - i] == 'o' || a[n - i] == 'u')
      c++;
    if (c == k) return 1;
  }
  return 0;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  cin >> n;
  long long int a[n], b[100002] = {0}, c;
  for (int i = 0; i < n; i++) cin >> a[i];
  for (long long int i = 0; i < n; i++) {
    c = min(i, n - 1 - i);
    if (a[i] - c <= 0) continue;
    b[a[i] - c]++;
  }
  m = *max_element(b, b + 100002);
  cout << n - m;
}
