S = input()
L = list(S)
D = {}
for i in L:
    if i in D.keys():
        D[i] += 1
    else:
        D[i] = 1
cnt_0 = 0
for key, value in D.items():
    if value == 2:
        cnt_0 += 1
    if value == 1:
        d = key
if cnt_0 == 6:
    print(d)
else:
    print("Impossible")