题解 | #DNA序列#

while True:
    try:
        line = input()
        n = int(input())
    except EOFError:
        break
    ret = ""
    m = 0
    for i in range(len(line) - n + 1):
        count = 0
        for c in line[i:i+n]:
            if c == 'G' or c == 'C':
                count += 1
                continue
        if count > m:
            m = count
            ret = line[i:i+n]
    print(ret)