s = io.read()
t = {}
for i = 1, 13 do
    a = string.sub(s, i, i)
    if(t[a] == nil) then t[a] = 1 else t[a] = t[a] + 1 end
end
tgt = nil
imp = false
for k, v in pairs(t) do
    if(v == 1) then
        if(tgt == nil) then tgt = k else imp = true end
    elseif(2 < v) then
        imp = true
    end
end
if(imp) then print("Impossible") else print(tgt) end