from collections import Counter

import sys


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    s = input()
    c = Counter(s)
    for k, v in c.items():
        if v == 1:
            ans = k
        if v not in [1, 2]:
            print('Impossible')
            return
    print(ans)



if __name__ == '__main__':
    main()