while True:
    try:
        words = input()
        s = sorted(set(words))// 用 set 去掉多余的数字，进行第一步按ACSII码来排序
        ss = sorted(s, key = lambda x:words.count(x),reverse=True)//通过key=lambda再按照次数进行排序
        print("".join(ss))
    except:
        break