(function (f, n, l, r)
	local a,ret={},0
	for i=l,r do a[f(i)]=0 end
	for _=1,n do
		local c=io.stdin:read(1)
		if a[c]==nil or a[c]==2 then ret=-1 break end
		if a[c]==1 then ret=1 end
		a[c]=a[c]+1
	end
	if ret==-1 then print("Impossible")
	elseif ret==1 then for k,v in pairs(a) do if v==0 then print(k) end end
	else for i=l,r do print(f(i)) end end
end)(string.char, 13, 97, 109)