from math import ceil

N,K,X,Y = [int(zz) for zz in input().split()]
A = [ceil((int(zz)-1)/K) for zz in input().split()]

A.sort()
tmp = 0
ans = 0
for i in range(N):
    if A[i]-tmp > 0:
        if X*(N-i) > Y:
            ans += Y*(A[i]-tmp)
        else:
            break
        tmp = A[i]
    A[i] = 0
if sum(A) > 0:
    ans += (sum(A)-(N-i)*tmp)*X
print(ans)