#!/usr/bin/env python3
import sys

def main():
    S = input()
    l = [0] * 26
    for ss in S:
        l[ord(ss) - ord("a")] += 1
    ans = []
    for i, ll in enumerate(l):
        if ll > 2:
            print("Impossible")
            return
        if ll == 1:
            ans.append(chr(i + ord("a")))

    if len(ans) == 1:
        print(ans[0])
    else:
        print("Impossible")
    return

if __name__ == '__main__':
    main()