import java.util.*;  public class Main2 { public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);  int N = sc.nextInt();  int d = sc.nextInt();  sc.nextLine();  List<Integer> dec = new ArrayList<>();  // HashSet<Integer> set = new HashSet<>();  int max = 0;  //双层矩阵  for (int i = 0; i < N; i++) { for (int j = 0; j < N; j++) { int m = sc.nextInt();  //set.add(m);  max = Math.max(max, m);  dec.add(m);  }
            sc.nextLine();  } int count = 0;  for (int num : dec) { if ((max - num) % d == 0) {
                count += (max - num) / d;  } else {
                count = -1;  break;  }
        }
        System.out.println(count);  }
}