S = input()
l = list(S)
st = set(l)

if len(st) != 7:
    print("Impossible")
else:
    l.sort()
    for i in range(0,len(l) -  1,2):
        if l[i] == l[i + 1]:
            pass
        else:
            if l.count(l[i]) == 1:
                print(l[i])
                break
            else:
                print(l[i + 1])
                break