题解 | #DNA序列#

from collections import defaultdict
def gc_ratio():
    dna = input()
    n = int(input())
    l = len(dna)
    left = 0
    count = defaultdict(lambda:0)
    while left<l-n+1:
        for i in range(left,left+n):
            if dna[i]=='C' or dna[i]=='G':
                count[left]+=1
        if count[left]==n:
            print(dna[left:left+n])
            return
        left+=1
    assist = list(count.items())
    assist.sort(key=lambda x:(-x[1],x[0]))
    print(dna[assist[0][0]:assist[0][0]+n])

gc_ratio()