content: package v103;
  
  
  import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.util.StringTokenizer;
  
  public class ChooseAndDivide_UVa10375 {
  
  	static double nCr(int n1, int k1, int n2, int k2)
  	{
  		if(n1 - k1 < k1)
  			k1 = n1 - k1;
  		if(n2 - k2 < k2)
  			k2 = n2 - k2;
  		double ret = 1.0;
  		for(int i = 1, j = 1; i <= k1 || j <= k2; ++i, ++j)
  		{
  			if(i <= k1)
  				ret = ret * (n1 - i + 1) / i;
  			if(j <= k2)
  				ret /= 1.0 * (n2 - j + 1) / j;
  			
  		}
  		
  		return ret;
  	}
  	public static void main(String[] args) throws IOException {
  		
  		Scanner sc = new Scanner(System.in);
  		PrintWriter out = new PrintWriter(System.out);
  		while(sc.ready())
  		{
  			int n1 = sc.nextInt(), k1 = sc.nextInt(), n2 = sc.nextInt(), k2 = sc.nextInt();
  			out.format("%.5f\n", nCr(n1, k1, n2, k2));
  		}
  		out.flush();
  	}
  
  	static class Scanner 
  	{
  		StringTokenizer st;
  		BufferedReader br;
  
  		public Scanner(InputStream s){	br = new BufferedReader(new InputStreamReader(s));}
  
  		public String next() throws IOException 
  		{
  			while (st == null || !st.hasMoreTokens()) 
  				st = new StringTokenizer(br.readLine());
  			return st.nextToken();
  		}
  
  		public boolean nxtEmpty() throws IOException
  		{
  			String line = br.readLine();
  			if(line.isEmpty())
  				return true;
  			st = new StringTokenizer(line);
  			return false;
  		}
  		public int nextInt() throws IOException {return Integer.parseInt(next());}
  		
  		public long nextLong() throws IOException {return Long.parseLong(next());}
  
  		public String nextLine() throws IOException {return br.readLine();}
  			
  		public boolean ready() throws IOException {return br.ready();}
  
  
  	}
  }
  