题解 | #DNA序列#

while True:
    try:
        s = input()
        n = int(input())
        dic = {}
        for i in range(len(s)-n+1):
            zi = s[i:i+n]
            
            a = zi.count('G') + zi.count('C')
            dic[zi] = a
        ls1 = []
        for i, j in dic.items():
            ls1.append(j)
        b = max(ls1)
        for i, j in dic.items():
            if j == b:
                print(i)
                break
    except:
        break