import sequtils,strutils

proc hantei(s : string):bool=
    const
        a = ord('a')
        m = ord('m')
    var
        cnt : array[a..m,int]
        
    for c in s:
        if ord(c) > m:
            return false
        else:
            cnt[ord(c)] += 1
            if cnt[ord(c)] > 2:
                return false
    for a in cnt:
        if a == 0:
            return false
    return true
        


var
    S = stdin.readline
    ans = newSeq[char](0)

for c in {'a'..'m'}:
    S &= c
    if hantei(S):
        ans.add(c)
    S = S[0..<S.high]

if ans.len>0:
    for c in ans:
        echo c
else:
    echo"Impossible"