n,k,x,y =map(int,input().split())#behoimi x behoma y
a=list(map(int,input().split()))
a.sort()
kaisu=[]
done=0
ans=0
for i in a:
    i-=1
    j=i//k
    if i%k!=0:
        j+=1
    kaisu.append(j)

for i in kaisu:
    i-=done
    if y<n*x:
        done+=i
        ans+=y*i
    else:
        ans+=x*i
    n-=1
print(ans)