#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> A(n);
  int ma = 0;
  for (int i = 0; i < n; i++) {
    cin >> A[i];
    ma = max(ma, A[i]);
  }
  map<int, vector<int> > make_pair;
  vector<int> req(n);
  req = A;
  int st = 1;
  for (int i = 0; i < n / 2; i++) {
    req[i] = st + i;
  }
  if (n % 2 != 0)
    req[n / 2] = req[n / 2 - 1] + 1;
  else
    req[n / 2] = req[n / 2 - 1];
  for (int i = n / 2 + 1; i < n; i++) {
    req[i] = req[i - 1] - 1;
  }
  for (int i = 1; i < n; i++) {
    if (A[i] > req[i]) {
      make_pair[req[i] - A[i]].push_back(i);
    }
    if (A[i] <= req[i]) {
      make_pair[req[i] - A[i]].push_back(i);
    }
  }
  int chngs = 0;
  chngs = (n - 1) - make_pair[0].size();
  int best = chngs;
  if (abs(A[0] - st) != 0) best++;
  for (st = 2; st <= ma; st++) {
    chngs = (n - 1) - make_pair[-(st - 1)].size();
    if (abs(A[0] - st) != 0) {
      chngs++;
    }
    best = min(best, chngs);
  }
  cout << best;
  return 0;
}
