#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, d, cot = 0;
  cin >> n >> m >> d;
  int arr[10004] = {0};
  int rn = n * m;
  for (int j = 1; j <= rn; j++) cin >> arr[j];
  sort(arr, arr + rn);
  int ind = arr[rn / 2];
  if (n * m == 1) {
    cout << 0;
    return 0;
  }
  for (int i = 1; i <= rn; i++) {
    if (abs(ind - arr[i]) % d != 0) {
      cout << -1;
      return 0;
    } else {
      cot += abs(ind - arr[i]) / d;
    }
  }
  cout << cot << endl;
  return 0;
}
