#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int f = 1, x = 0;
  char c = getchar();
  for (; !isdigit(c); c = getchar())
    if (c == '-') f = -1;
  for (; isdigit(c); c = getchar()) x = x * 10 + c - 48;
  return f * x;
}
int a, n, t[100005], s = 0;
int main() {
  n = read();
  for (int i = 1; i <= n; i++) {
    a = read();
    a -= min(i, n - i + 1);
    if (a >= 0) {
      t[a]++;
      s = max(s, t[a]);
    }
  }
  cout << n - s;
  return 0;
}
