s=list(input())
t=0
for i in range(13):
    x=s.count(chr(ord('a')+i))
    if 2<x:
        print('Impossible')
        exit(0)
    if x==2:
        t+=1
        if t==2:
            print('Impossible')
            exit(0)
if t==1:
    for i in range(13):
        if not s.count(chr(ord('a')+i)):
            print(chr(ord('a')+i))
            exit(0)
else:
    for i in range(13):
        print(chr(ord('a')+i))