#!/usr/bin/env fsharpi

open System;

let ParseIntN ( str : string ) =
    str.Split(' ')
    |> Array.map System.Int32.Parse

let ParseInt1 ( str : string ) =
    ParseIntN str |> fun arr -> arr.[0]

let ParseInt2 ( str : string ) =
    ParseIntN str |> fun arr -> (arr.[0],arr.[1])

let ParseInt3 ( str : string ) =
    ParseIntN str |> fun arr -> (arr.[0],arr.[1],arr.[2])

let ReadLinesN ( n : int ) =
    fun _ -> Console.ReadLine()
    |> Seq.initInfinite
    |> Seq.take n

let ReadLines =
    fun _ -> Console.ReadLine()
    |> Seq.initInfinite
    |> Seq.takeWhile ((<>) null)

let n = ParseInt1 <| Console.ReadLine()
let k = ParseInt1 <| Console.ReadLine()
let x = ParseInt1 <| Console.ReadLine()
let y = ParseInt1 <| Console.ReadLine()
let res = x * (min n k) + y * (max 0 (n-k))
printfn "%d" res
