public class TryMe {
public static void main(String[] args) {
  int a[] = {24, 35, 67, 89, 102, 134, 167, 189, 209, 289, 354, 396, 425};
  System.out.println(mystery(a, 13));
  int b[] = {35, 67, 89, 105, 135, 179};
  System.out.println(mystery(b, 6));
}
public static int mystery(int a[], int size){
   if (size == 1)
      return a[0];
   if (size == 2)
     return (a[0] - a[1]) / 2;
   int helper[] = new int[a.length];
   int k = 0;
   for (int j = 1; j < size - 1; j++){
     helper[k] = a[j];
     k--;
   }
  return mystery(helper, size - 2);