pai = list(input())

ans_char = ''
is_correct = True
for ch in pai:
    if pai.count(ch) == 1:
        if ans_char == '':
            ans_char = ch
        else:
            is_correct = False
            break

    elif pai.count(ch) == 2:
        pass
    
    elif pai.count(ch) >= 3:
        is_correct = False
        break

if is_correct:
    print(ans_char)
else:
    print('Impossible')