#-------------------------------------------------------------------
import sys
def p(*_a):
  _s=" ".join(map(str,_a))
  #print(_s)
  sys.stderr.write(_s+"\n")
#-------------------------------------------------------------------
import math

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

B = sorted(A)
C = [math.ceil((b-1)/K) for b in B]

p("B=",B)
p("C=",C)

hp = 1
ans = 0
for i in range(N):
  if B[i] <= hp: continue

  if (N-i)*X >= Y:
    ans += Y
    hp += K
    continue

  zan = B[i] - hp
  kai = math.ceil(zan / K)

  ans += kai * X
  B[i] -= kai * K


print(ans)