题解 | #DNA序列#

x = str(input())
x = list(x)
x.reverse()
x = "".join(x)

k = int(input())
r = -1
ans = ""

if k == len(x):
    r = x.count('G') + x.count('C')
    r = r/len(x)
    ans = x

# print(x[len(x)-k: len(x)])


for i in range(len(x)-k+1):
    # 这道题这里注意不要采用与k*r进行比较，否则就会出现问题
    if (x[i: i+k].count('C') + x[i: i+k].count('G'))/k >= r :
        ans = x[i: i+k]
        r = x[i: i+k].count('C') + x[i: i+k].count('G')
        r = r/k 

ans = list(ans)
ans.reverse()
ans = "".join(ans)

print(ans)