#!/usr/bin/env python

txt_input = input()
S = list(txt_input)

S.sort()
#print(S)
S_unique = list(set(S))

#print(len(S_unique))
if len(S_unique) == 7:
    for i in range(len(S)):
        if S.count(S[i]) >= 3:
            print('Impossible')
            break

            if S.count(S[i]) == 1:
                print(S[i])
                break
            else:
                pass
        else:
            pass
else:
    print('Impossible')