#include <bits/stdc++.h>   using namespace std;   const int MAX = 1e5 + 5;   const int mod = 1e9 + 7;   long long n, k, arr[MAX], ans = 0;   int main(int argc, char** argv) {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     cin >> n >> k;     if (n == 1) {       cout << 0;       exit(0);     }     bool moded = true;     for (int i = 0; i < n; i++) cin >> arr[i];     sort(arr, arr + n);     long long modu = arr[0] % k;     for (int i = 0; i < n; i++)       if (arr[i] % k != modu) moded = false;     if (moded)       for (int i = 0; i < n; i++) ans += (arr[i] - arr[0]) / k;     else       ans = -1;     cout << ans;     return 0;   }   