题解 | #DNA序列#

'''
解题思路：
将GC用数字1替换，其它用0替换，滑窗求和求极值，记录最大长度+开始位置
'''
s = input()
n = int(input())
s2 = s.replace('G','1')
s2 = s2.replace('C','1')
s2 = s2.replace('A','0')
s2 = s2.replace('T','0')
L = list(map(int,list(s2)))
#print(s)
#print(n)
#print(L)

i_max = 0
t_max = 0
t = 0
for i in range(len(L)-n+1):
    #print('i=',i)
    #print(L[i:i+n])
    t = sum(L[i:i+n])
    if t > t_max:
        t_max = t 
        i_max = i
print(s[i_max:i_max+n])