#!/usr/bin/env python

slist = list(input())

num1 = []
for s in set(slist):
    if slist.count(s) >= 3:
        print('Impossible')
        exit()
    elif slist.count(s) == 1:
        num1.append(s)

if len(num1) == 1:
    print(num1[0])
else:
    print('Impossible')