def build_complement(input_sequence):
    """
    :param input_sequence: str, allow users to input a sequence that is going to
    be built complement.
    :return: str, this function will return a sequence that is the complement of
    the input sequence.
    """
    output_sequence = ''
    for s in input_sequence:
        if s == 'A':
            output_sequence += 'T'
        elif s == 'T':
            output_sequence += 'A'
        elif s == 'G':
            output_sequence += 'C'
        else:
            output_sequence += 'G'

    return output_sequence