import java.util.*; import java.io.*; import java.math.*; public class No306 { public static void main(String[] args) { Scanner sc = new Scanner(System.in); int xa = sc.nextInt(); int ya = sc.nextInt(); int xb = sc.nextInt(); int yb = sc.nextInt(); System.out.println(1.0*yb-(1.0*yb-ya)/(1.0*xb+xa)*xb); } }