inputed_list = list(input())
count_list = []

for e in inputed_list:
    count_list.append(inputed_list.count(e))

count_1 = count_list.count(1)
count_2 = count_list.count(2)
if count_1 == 1 and count_2 == len(count_list) - 1:
   print(inputed_list[count_list.index(1)])
else:
    print('Impossible')