Find the maximum possible k-contiguous cumulative sum among all the elements in the given array.

If there is no such sum, return -1;

Example: {1,2,7,7,4,3,6}, 3

return 14 since 7 + 4 + 3 = 14 where 7, 4, 3 are unique elements in the array.

Using Java. Explain with details.