s = list(input())
ls = list('abcdefghijklm')

n = []
for i in ls:
    if i not in s:
        n.append(i)
if len(n) == 0:
    for i in ls:
        print(i)
elif len(n) == 1:
    print(n[0])        
        
else:
    print('Impossible')