str1 = input()
n = int(input())

flag = 0
index = 0
for i in range(len(str1)-n):
    s = str1[i:i+n] # 可以举个例子试一试
    CG = s.count('C') + s.count('G')
    if flag < CG / n:
        flag = CG/n
        index = i

print(str1[index:index+n])