#!/usr/bin/env python
txt_input = input()
S = list(txt_input)

S.sort()
#print(S)
count = 0
letter = ''

S_unique = list(set(S))

for i in range(len(S)):
    if S.count(S[i]) == 1:
        count = count + 1
        letter = S[i]
    else:
            pass
if count == 1:
    print(letter)
else:
    print('Impossible')