#include <bits/stdc++.h>
using namespace std;
long long a[1000000];
int main() {
  long long i, num, res = 0, n, k, rem, fl = 0;
  cin >> n >> k;
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  sort(a, a + n);
  for (i = 1; i < n; i++) {
    if ((a[i] - a[0]) % k != 0) {
      fl = 1;
      break;
    } else {
      rem = (a[i] - a[0]) / k;
      res += rem;
    }
  }
  if (fl) {
    cout << -1 << endl;
  } else
    cout << res << endl;
  return 0;
}
