#include <bits/stdc++.h>
using namespace std;
int cmpfunc(const void* a, const void* b) { return (*(int*)a - *(int*)b); }
int main() {
  int A, B, i;
  long long int sum = 0;
  int a[100001] = {0};
  cin >> A;
  cin >> B;
  for (i = 0; i < A; i++) {
    cin >> a[i];
  }
  qsort(a, A, sizeof(int), cmpfunc);
  for (i = 1; i < A; i++) {
    if ((a[i] - a[0]) % B == 0)
      sum = sum + (a[i] - a[0]) / B;
    else {
      sum = -1;
      break;
    }
  }
  cout << sum;
  return 0;
}
