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}
n = i ? N - i : 0

while n > 0 do
	if n * X > Y then
		m += Y
	else
		m += X * n
	end
	h += K
	i = a.bsearch_index{|x| x > h}
	n = i ? N - i : 0
end

puts m