题解 | #DNA序列# 循环遍历子序列，找出最大GC-Ratio输出对应基因

    try:
        DNA = input()                            # 记录DNA  
        k = int(input())                         # 记录序列长度
        l = len(DNA)                             # 记录DNA长度
        DNA_little = ''                          # 保存用于遍历的序列
        Max_DNA = ''                             # 保存GC-Ratio最高的序列   
        Max_ratio = 0                            # 保存GC-Ratio
        for i in range(l-k+1):                   
            DNA_little = DNA[i:i+k]              # 遍历子序列
            m = DNA_little.count('G')            
            n = DNA_little.count('C')
            ratio = m + n                        # 计算GC-Ratio
            if ratio > Max_ratio:
                Max_ratio = ratio                # 存储最大GC-Ratio
                Max_DNA = DNA_little             # 存储最大GC-Ratio对应的基因子序列
        print(Max_DNA)
    except:
        break