import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.Set;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final int xa = sc.nextInt();
		final int ya = sc.nextInt();
		final int xb = sc.nextInt();
		final int yb = sc.nextInt();
		
		int min_y = -1, max_y = -1;
		int min_y_x = -1, max_y_x = -1;
		if(ya < yb){
			min_y = ya;   max_y = yb;
			min_y_x = xa; max_y_x = xb;
		}else{
			min_y = yb;   max_y = ya;
			min_y_x = xb; max_y_x = xa;
		}
		
		if(max_y == min_y){
			System.out.println(min_y);
		}else{
			System.out.printf("%.10f\n", min_y + ((max_y - min_y) * min_y_x / (double)(max_y_x + min_y_x)));
		}
		
	}

}