import sys

def main():
    S = input()
    one_char = []
    list_S = list(S)
    for s in set(list_S):
        if list_S.count(s) == 2:
            continue
        elif list_S.count(s) == 1:
            one_char.append(s)
        else:
            print("Impossible")
            sys.exit()
    #print(one_char)
    if len(one_char) == 1:
        print(one_char[0])
    else:
        print("Impossible")

if __name__ == '__main__':
    main()