package main

import (
	"fmt"
	"bufio"
	"os"
	"sort"
)

func main() {
	r := bufio.NewReader(os.Stdin)
	w := bufio.NewWriter(os.Stdout)
	defer w.Flush()
	var N, K, X, Y int
	fmt.Fscan(r, &N, &K, &X, &Y)
	a := make([]int, N)
	for i := 0; i < N; i++ {
		fmt.Fscan(r, &a[i])
	}
	sort.Ints(a)
	h, m := 1, 0
	i := sort.Search(N, func(i int) bool {return a[i] > h})
	var n, minh int
	if i < N {
		n = N - i
		minh = a[i] - h
	} else {
		n = 0
		minh = 0
	}

	for n > 0 {
		var v int
		if n * X > Y {
			v = Y
		} else {
			v = n * X
		}
		p := (K - 1 + minh) / K
		m += p * v
		h += K * p
		i = sort.Search(N, func(i int) bool {return a[i] > h})
		if i < N {
			n = N - i
			minh = a[i] - h
		} else {
			n = 0
			minh = 0
		}		
	}
	fmt.Fprintln(w, m)
}