def dna(sequence):
    """Finds complementary DNA sequence
    
    Arguments:
        sequence {String} -- A DNA sequence, e.g `ATTGC`
    
    Returns:
        [Stringe] -- Complementatry DNA sequence to given one
    """
    dna = []
    dictionary = {'A': 'T', 'T':'A', 'G': 'C', 'C': 'G'}
    for n in sequence:
        dna.append(dictionary.get(n))
    return ''.join(dna)