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.io.*;
import java.util.*;
public class Asd {
static PrintWriter w=new PrintWriter(System.out);
static FastScanner s=new FastScanner();

	public static void main(String[] args) {
		
		//int t=s.nextInt();
              int t=1;
		while(t-->0)
                {
		    solve();
		}
	       w.close();
        }
        /* Function to print all the permutations of the string
        static String swap(String str, int i, int j)
        {
        char ch;
        char[] array = str.toCharArray();
        ch = array[i];
        array[i] = array[j];
        array[j] = ch;
        return String.valueOf(array);
       }
    
      static void permute(String str,int low,int high)
    {
        if(low == high)
            list.add(Long.parseLong(str));
 
        int i;
        for(i = low; i<=high; i++){
            str = swap(str,low,i);
            permute(str, low+1,high);
            str = swap(str,low,i);
        }
    }
        use  permute(str2,0,str2.length()-1); to perform combinations
        */
           public static void solve()
        {
          int a=s.nextInt();
          int b=s.nextInt();
          int c=s.nextInt();
          int d=s.nextInt();
               
           double p1=((double)a/b);
           double p2=(double)c/d;
          double p11=1-p1;
           double p22=1-p2;
           double sum=p1;
           for(int i=1;i<=100000;i+=1)
           {
               sum+=(Math.pow(p11,i)*Math.pow(p22,i)*p1);
           }
               w.println(sum);
        }          
        static int noofdivisors(int n)
        {
            //it counts no of divisors of every number upto number n
            
            int arr[]=new int[n+1];
              for(int i=1;i<=(n);i++)
			for(int j=i;j<=(n);j=j+i)
				arr[j]++;
             return arr[0];
        }
           
        
       
        static char[] reverse(char arr[])
        {
            char[] b = new char[arr.length]; 
        int j = arr.length; 
        for (int i = 0; i < arr.length; i++) { 
            b[j - 1] = arr[i]; 
            j = j - 1; 
        }
        return b;
        }
        
        
	static long factorial(int n)
        {
            long su=1;
            for(int i=1;i<=n;i++)
                su*=(long)i;
            return su;
        }
	static class FastScanner {
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer("");
		public String next() {
			while (!st.hasMoreElements())
				try {
					st=new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}
		int[] readArray(int n) {
			int[] a=new int[n]; 
			for (int i=0; i<n; i++) a[i]=nextInt();
			return a;
		}
		
		int nextInt () {
			return Integer.parseInt(next());
		}
                long nextLong () {
			return Long.parseLong(next());
		}
                double nextDouble () {
			return Double.parseDouble(next());
		}
	}

}