#!/usr/bin/env python3
import sys

def main():
    S = input()
    l = [0] * 13
    for ss in S:
        l[ord(ss) - ord("a")] += 1
    # 0 2 1つ
    # 1 全て
    if l == [1] * 13:
        print(*"abcdefghijklm", sep="\n")
        return
    if sorted(l) == [0] + [1] * 11 + [2]:
        print(chr(l.index(0) + ord("a")))
        return
    print("Impossible")
    return
        

if __name__ == '__main__':
    main()