fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)


fun member x nil = false
  | member x (h::tl) =
    if x = h
    then true
    else member x tl


val () =
    let
        val a = readInt ()
        val b = readInt ()
        val c = readInt ()
        val d = readInt ()

        val all = (b - a + 1) * (d - c + 1)
        val ab = List.tabulate (b - a + 1, fn x => a + x)
        val cd = List.tabulate (d - c + 1, fn x => c + x)
        val sameValues =
            List.filter
                (fn x =>
                    member x cd)
                ab
        val ans = all - (List.length sameValues)
    in
        print (Int.toString ans ^ "\n")
    end