N, K, X, Y = gets.split(" ").map{|s| s.to_i}
a = gets.split(" ").map{|s| s.to_i}

a.sort!

h = 1
m = 0

i = a.bsearch_index{|x| x > h}
if i then
	n = N - i
	minh = a[i] - h
else
	n = 0
	minh = 0
end

while n > 0 do
	v = n * X > Y ? Y : n * X
	p = (K - 1 + minh) / K
	m += p * v
	h += K * p
	i = a.bsearch_index{|x| x > h}
	if i then
		n = N - i
		minh = a[i] - h
	else
		n = 0
		minh = 0
	end
end

puts m