[<AutoOpen>]
module Program

open System

let read f = stdin.ReadLine() |> f
let reads f = stdin.ReadLine().Split() |> Array.map f

[<EntryPoint>]
let main argv =
    let [|hh;ww|] = reads int
    let [|h;w|] = reads int
    printfn "%i" ((hh - h) * (ww - w))
    0