HJ63 DNA序列

st = input()
n = int(input())

max_ratio = 0
ratio = 0
res =''
for i in range(0,len(st)-n+1):
    s = st[i:i+n]
    ratio = s.count('C')+s.count('G')
    if ratio > max_ratio:
        max_ratio = ratio
        res = s
print(res)