S = input()
l = list(S)

ck = 0


l.sort()
l1 = []
flg = 0
for i in l:
    ck = l.count(i)
    if ck == 1:
        l1.append(i)
    elif ck == 2:
        pass
    else:
        flg = 1
        break

if len(l1) == 1 and flg == 0:
    print(l1[0])
else:
    print("Impossible")