from math import ceil
n,k,x,y=map(int,input().split())
a=sorted([*map(lambda x:ceil((int(x)-1)/k),input().split())])
ans,m,b,c=0,0,n,0
while b>0:
    m=a[-b]-c
    ans+=y*m if y<b*x else b*x*m
    c=a[-b]
    while a[-b]-c<=0 and b>0:b-=1
print(ans)