import sequtils,strutils
var
    s = stdin.readline
    cnt : array['a'..'z', int]
    a : int
    ans : char
for c in s:
    cnt[c] += 1
    
for c in 'a'..'z':
    if cnt[c] > 2:
        a = 100
    elif cnt[c] == 1:
        a += 1
        ans = c
if a == 1:
    echo ans
else:
    echo "Impossible"