题解 | #DNA序列#

while True:

try:
    str_input = input()
    n = int(input())
    
    gc_ratio_list = [0] * (len(str_input) - n + 1)
    
    for i in range(len(gc_ratio_list)):
        g_count = str_input[i: i + n].count('G')
        c_count = str_input[i: i + n].count('C')
        gc_ratio_list[i] = (g_count + c_count) / len(str_input)
        
    print(str_input[gc_ratio_list.index(max(gc_ratio_list)): gc_ratio_list.index(max(gc_ratio_list)) + n])
    
    
except:
    break