One element removed

There are two lists, list X and list Y. Both lists contain integers from -1000 to 1000 and are identical to each other except that one integer is removed in list Y that exists in list X.

Write a function one_element_removed that takes in both lists and returns the integer that was removed in $O(1)$ space and $O(n)$ time without using the python set function.

Example:

Input:

list_x = [1,2,3,4,5]
list_y = [1,2,4,5]

one_element_removed(list_x, list_y) -> 3