def calculate_hamming_distance(first_dna, second_dna):
    """
    Calculates the Hamming distance between first_dna and second_dna.

    Args:
        first_dna (str): DNA string.
        second_dna (str): DNA string.

    Returns:
        int: the Humming distance.
    """
    hamming_distance = 0

    for index in range(len(first_dna)):
        if first_dna[index] != second_dna[index]:
            hamming_distance += 1

    return hamming_distance