target = input()
dict = {}
for char in target:
    if char in dict:
        dict[char] +=1
    else:
        dict[char] = 1
print(dict)
if len(dict)==7:
    dict2 = {v:k for k, v in dict.items()}
    if len(dict2)==2:
        print(dict2[1])
    else:
        print('Impossible')
else:
    print('Impossible')