n,k,x,y=map(int,input().split())
a=[int(i)-1 for i in input().split()]
idx=-1

for i in range(n):
  if y<=x*(i+1):
    idx=i+1
    break
a.sort()
from math import ceil
for i in range(n):
  a[i]=ceil(a[i]/k)
# print(a)

if sum(a)<y:
  print(sum(a)*x)
  exit(0)

if idx==-1:
  print(a[-1]*y)
  exit(0)


idx-=1
v=a[-1-idx]
a=[0 if int(aa)-v<0 else int(aa)-v for aa in a]
# print(a)
print(sum(a)*x+v*y)