Scanf.scanf "%d %d %d %d" (fun a b c d ->
    let rec loop_i i acc =
        let rec loop_j j acc =
            if j > d then acc else
                let acc = if i <> j then acc + 1 else acc in
                loop_j (j + 1) acc
        in
        if i > b then acc else loop_i (i + 1) (loop_j c acc)
    in
    loop_i a 0 |> Printf.printf "%d\n"
)