题解 | #DNA序列#

def cal(s):
    n = len(s)
    tmp = 0
    for i in s:
        if i =='G' or i=='C':
            tmp+=1
    return tmp/n

n = input().strip()
m = int(input())
re = []

for i in range(len(n)-m+1):
    t = n[i:i+m]
    re.append([t,cal(t)])

re = sorted(re,key=lambda x:x[1],reverse=True)
print(re[0][0])