题解 | #DNA序列#

string = input()
n = int(input())
max_count = 0
result = ''

for i in range(len(string) - n + 1):
    substring = string[i:i+n]
    count = 0
    for char in substring:
        if char == "G" or char == "C":
            count += 1
    if count > max_count:
        max_count = count
        result = substring

print(result)