open System

let solve a b c d =
    let cdSet = [c .. d] |> Set.ofList
    let cdSetSize = d - c + 1 
    let cnts = [a .. b] |> Seq.countBy(fun x -> Set.contains x cdSet)
    let getCount flag = Seq.tryFind(fun (k, _) -> k = flag) >> Option.map snd >> Option.defaultValue 0
    let t1 = cnts |> getCount false 
    let t2 = cnts |> getCount true
    cdSetSize * t1 + (cdSetSize - 1) * t2
    
let a, b, c, d =
    let t = Console.ReadLine().Split() |> Array.map int
    t.[0], t.[1], t.[2], t.[3]

solve a b c d
|> Console.WriteLine