def distance(strand_a, strand_b):
    length_sum = len(strand_a) - len(strand_b)
    if (strand_a == strand_b):
        return 0
    if (length_sum != 0):
        raise ValueError("Not the same distance")
    distance = 0
    for i, letter in enumerate(strand_a):
        if (letter != strand_b[i]):
            distance += 1
    return distance