s = input()
d = {'a':0,'b':0,'c':0,'d':0,'e':0,'f':0,'g':0,'h':0,'i':0,'j':0,'k':0,'l':0,'m':0}

try:
    for i in range(len(s)):
        k = s[i:i+1]
        d[k] += 1

    if max(d.values()) == 1:

        for k in d.keys():
            print(k)
    elif max(d.values()) == 2:

        print(min(d, key=d.get))
    else:
        print('Impossible')
except:
    print('Impossible')