nums = [0 for _ in range(26)]
for c in input():
    nums[ord(c) - 97] += 1
ans = ''
for i, num in enumerate(nums):
    if num > 2:
        print('Impossible')
        exit()
    elif num == 1:
        if not ans == '':
            print('Impossible')
            exit()
        else:
            ans = chr(i + 97)
print(ans)