# coding: utf-8
# Your code here!
import sys
read = sys.stdin.read
readline = sys.stdin.readline

n,k,x,y, *a = map(int,read().split())

c = y//x
a.sort()
a = [(i-1+k-1)//k for i in a]


ans = 0
idx = max(n-1-c,-1)
if idx >= 0: v = a[idx]
else: v= 0
ans += v*y

for i in a[idx+1:]:
    ans += (i-v)*x

print(ans)