Counting Point Mutations

Given two strings s and t of equal length, the Hamming distance between s and t, denoted d_H(s, t), is the number of corresponding symbols that differ in s and t.

Example: The Hamming distance between these two strings below is 7. Mismatched symbols are colored red.

                GAGCCTACTAACGGGAT
                CATCGTAATGACGGCCT
            

Given: Two DNA strings s and t of equal length (not exceeding 1 kbp).

Return: The Hamming distance d_H(s, t).