Write a function
that given two vectors X and Y in increasing order, returns the number of common elements in the two vectors, that is, the number of integer numbers a such that a=X[i]=Y[j] for any i and j.
The two vectors are in strictly increasing order.
common_elements([3,5,7,8], [2,3,7,9,10]) = 2.
common_elements([1,2,3,4,5], [3,4,5,6,7,8]) = 3.
common_elements([1,2,3,4,5], [8,9]) = 0.