Problem: SmallR is an archer. SmallR is taking a match of archer with Zanoes. They try to shoot in the target in turns, and SmallR shoots first. The probability of shooting the target each time is <image> for SmallR while <image> for Zanoes. The one who shoots in the target first should be the winner.

Output the probability that SmallR will win the match.

Input

A single line contains four integers <image>.

Output

Print a single real number, the probability that SmallR will win the match.

The answer will be considered correct if the absolute or relative error doesn't exceed 10 - 6.

Examples

Input

1 2 1 2


Output

0.666666666667

Solution (in JAVA):

Solution:
import java.util.Scanner;


public class p312B {

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		double a = in.nextDouble();
		double b = in.nextDouble();
		double c = in.nextDouble();
		double d = in.nextDouble();
		double q = (1- (a/b)) * (1 - (c/d));
		
		double ans = (a/b) / (1-q);
		
		System.out.println(ans);
	}

}