import java.io.*;
import java.util.*;
import java.math.*;

class Main {
	
	public static void out (Object o) {
		System.out.println(o);
	}
	
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
		
		String[] line = br.readLine().split(" ");
		int xa = Integer.parseInt(line[0]);
		int ya = Integer.parseInt(line[1]);
		line = br.readLine().split(" ");
		int xb = Integer.parseInt(line[0]);
		int yb = Integer.parseInt(line[1]);
		
		out(yb + xb * ((double)(ya - yb) / (xa + xb)));
	}
	
}