# coding: utf-8

def main():
    s = input()
    d = count(s)
    e = list(filter(lambda t: t[1] != 2, d.items()))
    if len(e) == 1 and e[0][1] == 1:
        print(e[0][0])
    else:
        print('Impossible')


def count(s):
    return { c: len(list(filter(lambda x: x == c, s))) for c in set(s) }


if __name__ == '__main__':
    main()