#!/usr/bin/env python
import fileinput
import itertools
import numpy as np
from collections import Counter


#print(fileinput.input())
txt_input = input_line()

for line in fileinput.input():
    line = line.replace('\n','')
    txt_input.append(line)

#print(txt_input, end='')

#標準入力からの取り込み
S = list(txt_input[0])

S.sort()

#print(S)
count = 0
lst =[]
letter = ''

S_unique = list(set(S))
if len(S) - len(S_unique) == 6:
    dct = Counter(S)
    search_letter = 1
    for name, age in dct.items():
        if age == search_letter:
            print(name)
else:
    print('Impossible')