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
