def get_common_bit(readings, index):
    """
    Returns the most common bit at index index in list readings
    :param readings: List:str
    :param index: int
    :return:
    """
    zeros, ones = 0, 0
    for reading in readings:
        if reading[index] == '0':
            zeros += 1
        elif reading[index] == '1':
            ones += 1
    if zeros > ones:
        return '0'
    elif ones > zeros:
        return '1'
    else:
        return '-1'