N,K,X,Y = map(int,input().split())
A = list(map(int,input().split()))
A.sort(reverse = True)
import sys

if Y <= X:
    d = A[0] - 1
    ans = (d + K - 1) // K * Y
    print(ans)
    exit()
q = (Y + X - 1) // X
if q > N:
    qq = 0
    ans = 0
    q = N + 1
else:
    d = A[q - 1] - 1
    qq = (d + K - 1) // K
    ans = qq * Y
for i in range(q - 1):
    d = A[i] - 1 - qq * K
    ans += (d + K - 1) // K * X
print(ans)