Scanf.scanf "%d %d" (fun n l ->
    let a = Array.init n (fun _ -> Scanf.scanf " %d" (fun a -> a)) in
    let rec loop i =
        if i = n - 1 then (-1) else
            if a.(i) + a.(i + 1) >= l then i else loop (i + 1)
    in
    let k = loop 0 in
    if k < 0 then print_endline "Impossible" else (
        print_endline "Possible";
        for j = 0 to k - 1 do
            Printf.printf "%d\n" (j + 1)
        done;
        for j = n - 2 downto k + 1 do
            Printf.printf "%d\n" (j + 1)
        done;
        Printf.printf "%d\n" (k + 1)
    )
)