题解 | #DNA序列#

while True:
    try:
        dna = input()
        num = int(input())
    except:
        break
    else:
        ratio = 0.0
        new_dna =[]
        for i in range(len(dna)-num+1):
            sub_dna = dna[i:i+num]
            num_c = sub_dna.count('C')
            num_g = sub_dna.count('G')
            new_ratio = (num_c+num_g)/num
            if new_ratio > ratio:
                ratio = new_ratio
                new_dna=sub_dna
        print(new_dna)