题解 | #DNA序列#

import sys

while True:
    try:
        line=input()
        num=int(input())
        
        result=[]
        for i in range(len(line)-num+1):
            freq=line[i:i+num].count('C')+line[i:i+num].count('G')
            result.append(freq)
        
        loc=result.index(max(result))
        print(line[loc:loc+num])
        
    except:
        break