题解 | #DNA序列#

# 20240920
while True:
    try:
        s = input()
        N = int(input())
        ls = []
        for i in range(len(s)):
            ls.append(s[i:i+N])
        lst = []
        for i in ls:
            if len(i) == N:
                lst.append(i)

        #print(ls)
        ls1 = []
        for i in lst:
            ls1.append((i.count('G')+i.count('C'))/len(i))
        #ls1.sort(reverse=True)
        print(ls[ls1.index(max(ls1))])
    except:
        break