#!/usr/bin/env python3
import sys

def main():
    S = input()
    l = [0] * 7
    for ss in S:
        if ord(ss) - ord("a") > 6:
            print("Impossible")
            return
        l[ord(ss) - ord("a")] += 1
    if sorted(l) == [1] + [2] * 6:
        print(chr(l.index(1) + ord("a")))
    else:
        print("Impossible")

if __name__ == '__main__':
    main()