#! /usr/bin/env python3

A = sorted(input())

l = []
for i in 'abcdefghijklm':
    if not i in A : l += [i]
if l == []:
    for i in 'abcdefghijklm':
        print(i)
elif len(l) < 2:
    print(l[0])
else:
    print('Impossible')