from math import floor,ceil

N,K,X,Y=map(int,input().split())
A=list(map(int,input().split()))
D=set()

B=[]
for i in range(N):
    x=ceil((A[i]-1)/K)
    B.append(x)

B.sort()

T=min(floor(Y/X),N)

if T==N:
    print(sum(B)*X)
else:
    P=B[N-1-T]
    S=P*Y
    for b in B:
        S+=max(0,b-P)*X

    print(S)