def hamming_distance(s1, s2):
    if len(s1) != len(s2):
        print("Strings must be of equal length")
        return -1

    count = 0
    strings_to_compare = list(zip(s1, s2))

    for i in range(len(strings_to_compare)):
        for j in range(i, i + 1):
            if strings_to_compare[i][0] != strings_to_compare[j][1]:
                count += 1

    return count