import java.util.*;
import java.math.*;
import java.text.*;

public class Main {
    private static Scanner sc = new Scanner(System.in);
    public static void main(String[] args) throws Exception {
        float xa = sc.nextFloat();
        float ya = sc.nextFloat();
        float xb = sc.nextFloat();
        float yb = sc.nextFloat();
        
        float diff = (yb-ya)*(xa/(xb+xa));
        
        System.out.println(ya + diff);
    }
}