def isdna (s):
    dna = 'ACGT'
    for n in s:
        if n not in dna:
            return False
    return True

def similar_dna(s1, s2):
    n = 0
    for i in range(len(s1)):
        if s1[i] == s2[i]:
            n = n + 1
    return n/len(s1) * 100