text=input("")

chars={}
for i in range(13):
    char=text[i]
    if char in chars:
        chars[char]+=1
    else:
        chars[char]=1

if len(chars)==7:
    one=0
    result=""
    other=0
    clist=chars.keys()
    for i,v in enumerate(chars):
        if chars[v]==1:
            one+=1
            result=v
        elif chars[v]!=2:
            other+=1
    if other!=0:
        result="Impossible"
    elif one!=1:
        resule="Impossible"
else:
    result="Impossible"
print(result)