#coding:utf-8

import collections


def main():
    s_list = list(input())
    s_dict = collections.Counter(s_list)
    key = [k for k, v  in s_dict.items() if v == 1 ]
    print(key[0]) if len(s_dict) == 7 and max(s_dict.values()) == 2 else print('Impossible')

if __name__ == '__main__':
    main()