题解 | #DNA序列#

while 1:
    try:
        DNA_str = input()
        N = int(input())

        mark = [[], []]

        for i in range(len(DNA_str) - N + 1):
            mark[0].append(DNA_str[i:i+N])
            mark[1].append(mark[0][i].count('C') + mark[0][i].count('G'))

        print(mark[0][mark[1].index(max(mark[1]))])
    except:
        break