题解 | #DNA序列#

a=input().strip()
num1 = int(input().strip())
table1 = {}
import sys
if num1 == len(a):
    print(a)
    sys.exit()
for i in range(0,len(a)-num1):
    count1 = 0
    for j in a[i:i+num1]:
        if j == "G" or j == "C":
            count1 +=1
    table1[(a[i:i+num1])] = count1/len(a)
resault1 = sorted(table1.items(),key=lambda x:x[1],reverse=True)
print(resault1[0][0])