# -*- coding: utf-8 -*-
"""
No.587 七対子
https://yukicoder.me/problems/no/587

"""
import sys
from sys import stdin
from collections import Counter
input = stdin.readline


def solve(S):
    ans = None
    chars = list(S)
    for c in Counter(chars).most_common():
        if c[1] > 2:
            return 'Impossible'
        if c[1] == 1:
            if not ans is None:
                return 'Impossible'
            ans = c[0]
    return ans


def main(args):
    S = input().strip()
    ans = solve(S)
    print(ans)


if __name__ == '__main__':
    main(sys.argv[1:])