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.*;
import java.lang.*;
import java.io.*;

	public class FastIO{
			
		BufferedReader br;
		StringTokenizer st;
		
		public FastIO(){ // constructor
			
			br = new BufferedReader(new InputStreamReader(System.in));
		}
		
		String next(){
			
			while (st == null || !st.hasMoreElements()){
				try{
					st = new StringTokenizer(br.readLine());
				}
				
				catch (IOException e){
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}
		
		int nextInt(){
			
			return Integer.parseInt(next());
		}
		
		long nextLong(){
			
			return Long.parseLong(next());
		}
		
		double nextDouble(){
			return Double.parseDouble(next());
		}

		String nextLine(){
			
			String str = "";
			try{
				
				str = br.readLine();
			}
			catch (IOException e){
				
				e.printStackTrace();
			}
			return str;
		}
	
	public static void main(String[] args){
		
		FastIO io = new FastIO();
		
		// int n = io.nextInt(); -> for int
		// double d = io.nextDouble(); -> for double
		// String ch = io.next(); -> for one word
		// String str = io.nextLine(); -> for the entire line
		
		double a = io.nextDouble();
		double b = io.nextDouble();
		
		double c = io.nextDouble();
		double d = io.nextDouble();
		
		double r = (1 - (a / b)) * (1 - (c / d));
		
		double ans = (a / b) / (1 - r);
		
		System.out.println(ans);
	}
}