#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e15;
long long arr[100003];
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%lld", &arr[i]);
  if (n == 1 || n == 2)
    printf("0\n");
  else {
    long long res = INF;
    for (int i = -1; i <= 1; i++) {
      for (int j = -1; j <= 1; j++) {
        long long a = arr[0] + i;
        long long b = (arr[1] + j) - (arr[0] + i);
        long long cnt = abs(i) + abs(j);
        for (int k = 2; k < n; k++) {
          if (abs(a + (k * b) - arr[k]) > 1)
            cnt = INF;
          else
            cnt += abs(a + (k * b) - arr[k]);
        }
        res = min(res, cnt);
      }
    }
    if (res <= 1e9)
      printf("%lld\n", res);
    else
      printf("-1\n");
  }
}
