c++++++++++++++++++++++++++++

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#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=1e6+5;
int a[N];
signed main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int n,k,getMax=0;
	cin>>n>>k;
	for(int i=1;i<=n;i++){
		cin>>a[i];getMax=max(getMax,a[i]);
	}
	int ans=0;
	for(int i=1;i<=n;i++){
		if(getMax!=a[i]&&(getMax-a[i])%k!=0){
			cout<<-1;
			return 0;
		}
		else if(getMax!=a[i]&&(getMax-a[i])%k==0){
			ans+=(getMax-a[i])/k;
		}
	}
	cout<<ans;
}

#include <bits/stdc++.h>
using namespace std;
long long n, a[105], k, s = 0;
int main()
{
  ios_base::sync_with_stdio(0);
  cin >> n >> k;
  for (long long i = 1; i <= n; ++i) cin >> a[i];
  if (k <= 0) return cout << "-1" , 0;
  long long mx = *max_element(a + 1, a + 1 + n);
  for (long long i = 1; i <= n; ++i)
  {
    if (mx > a[i])
    {
      long long d = mx - a[i];
      s += (d + k - 1) / k;    
    }
  }
  if (s > 0) cout << s;
  else cout << "-1";
  return 0;
}