#! ruby
# yukicoder My Practice
# author: Leonardone @ NEETSDKASU

A, B, X, Y = gets.chomp.split.map(&:to_f)

case A * Y <=> B * X
when -1
	pust (Y + A * Y / B) 
when 0
	puts (X + Y)
when 1
	puts (X + B * X / A)
end