def updiv(a,b):
    if a % b == 0:
        return a // b
    else:
        return a // b + 1

N,K,X,Y = map(int,input().split())

A = list(map(int,input().split()))

lis = []

for i in range(N):

    lis.append( updiv(A[i]-1 , K ))

ans = 0
lis.append(0)
lis.sort()

for i in range(N):

    ans += (lis[i+1]-lis[i]) * min(Y , (N-i) * X)

print (ans)