import sys
input = sys.stdin.readline
from collections import *

N, K, X, Y = map(int, input().split())
A = list(map(int, input().split()))
cnt = [0]*N

for i in range(N):
    cnt[i] = (A[i]+K-2)//K

cnt.sort()
ans = 0
prev = 0

for i in range(N):
    ans += (cnt[i]-prev)*min((N-i)*X, Y)
    prev = cnt[i]

print(ans)