#include <bits/stdc++.h>
using namespace std;
int N;
int A[100005];
int G[100005];
int m;
int main() {
  scanf("%d", &N);
  for (int i = 0; i < N; ++i) scanf("%d", &A[i]);
  for (int i = 0; i < N; ++i) {
    if (i < N / 2) {
      if (A[i] - i >= 1) ++G[A[i] - i];
    } else {
      if (A[i] - (N - i - 1) >= 1) ++G[A[i] - (N - i - 1)];
    }
  }
  m = 0;
  for (int i = 0; i <= 1e5; ++i)
    if (G[i] >= m) m = G[i];
  printf("%d", N - m);
  return 0;
}
