题解 | #DNA序列#

while True:
    try:
        str0 = input()
        n = int(input())
        x = str0.count('A') + str0.count('C') + str0.count('G') + str0.count('T')
        if 1 <= len(str0) <= 1000 and 0 < n <= len(str0) and str0.isupper() and x == len(str0):
            arr0 = list(str0)
            dic = {}
            for i in range(len(arr0)-n+1):
                str1 = ''
                for x in range(i, i + n):
                    str1 += arr0[x]
                sm=str1.count('C')*0.5+str1.count('G')*0.5
                if sm not in dic.keys():
                    dic[sm]=str1+','
                else:
                    dic[sm]=dic[sm]+str1+','
            arr1=[]
            for x in dic.keys():
                arr1.append(x)
            arr1.sort(reverse=True)
            print(dic[arr1[0]].split(',')[0])

        else:
            continue
        break
    except:
        break