#include <bits/stdc++.h>
using namespace std;
const int iinf = 1e9 + 7;
const long long linf = 1ll << 60;
const double dinf = 1e60;
template <typename T>
inline void scf(T &x) {
  bool f = 0;
  x = 0;
  char c = getchar();
  while ((c < '0' || c > '9') && c != '-') c = getchar();
  if (c == '-') {
    f = 1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    x = x * 10 + c - '0';
    c = getchar();
  }
  if (f) x = -x;
  return;
}
template <typename T1, typename T2>
void scf(T1 &x, T2 &y) {
  scf(x);
  return scf(y);
}
template <typename T1, typename T2, typename T3>
void scf(T1 &x, T2 &y, T3 &z) {
  scf(x);
  scf(y);
  return scf(z);
}
template <typename T1, typename T2, typename T3, typename T4>
void scf(T1 &x, T2 &y, T3 &z, T4 &w) {
  scf(x);
  scf(y);
  scf(z);
  return scf(w);
}
inline char mygetchar() {
  char c = getchar();
  while (c == ' ' || c == '\n') c = getchar();
  return c;
}
template <typename T>
void chkmax(T &x, const T &y) {
  if (y > x) x = y;
  return;
}
template <typename T>
void chkmin(T &x, const T &y) {
  if (y < x) x = y;
  return;
}
void TZL();
void RANK1();
int main() {
  TZL();
  RANK1();
  return 0;
}
void TZL() {
  long long k;
  int n;
  vector<long long> a;
  a.clear();
  scf(n, k);
  for (int i = 0, _end_ = (n); i < _end_; ++i) {
    long long x;
    scf(x);
    a.push_back(x);
  }
  sort(a.begin(), a.end());
  long long ans = 0;
  for (int i = 1; i < n; ++i) {
    long long x = a[i] - a[0];
    if (x % k != 0) {
      puts("-1");
      return;
    }
    ans += x / k;
    ;
  }
  cout << ans << endl;
}
void RANK1() {}
