#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a[100100];
  vector<vector<int> > b1;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  if (n < 3) {
    cout << 0;
    return 0;
  }
  for (int i = a[1] - 1; i <= a[1] + 1; i++) {
    for (int j = a[2] - 1; j <= a[2] + 1; j++) {
      vector<int> a1(n + 1, 0);
      a1[1] = i;
      a1[2] = j;
      int k = j - i, b = 0;
      for (int j = 3; j <= n; j++) {
        a1[j] = a1[j - 1] + k;
        if (abs(a1[j] - a[j]) > 1) {
          b = 1;
          break;
        }
      }
      if (b == 1) continue;
      b1.push_back(a1);
    }
  }
  int k = 0, min1 = 1000000;
  for (int i = 0; i < b1.size(); i++) {
    k = 0;
    for (int j = 1; j <= n; j++) {
      if (a[j] != b1[i][j]) k++;
    }
    min1 = min(k, min1);
  }
  if (b1.size() == 0)
    cout << -1;
  else
    cout << min1;
}
