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.*;
import static java.lang.Math.*;
public class ProblemB {
    void run() {
        FastScanner sc = new FastScanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int d = sc.nextInt();
		double p = ((double) a) / b;
		double q = ((double) c) / d;
		double ans = p / (1 - (1-p) * (1-q));
		p("%.7f\n", ans);
    }

    boolean debug = false;
    void p(String f, Object...params) {
        System.out.printf(f, params);
    }
    void d(Object...params) {
        if (debug) {
            p("DEBUG: %s\n", Arrays.deepToString(params));
        }
    }
    static void die() {
        throw new RuntimeException();
    }
    void timeout() {
        for (int x = 0; ; ++x);
    }
    String join(Collection<?> s, String delimiter) {
        StringBuilder builder = new StringBuilder();
        Iterator<?> iter = s.iterator();
        while (iter.hasNext()) {
            builder.append(iter.next());
            if (iter.hasNext()) {
                builder.append(delimiter);
            }
        }
        return builder.toString();
    }
    public ProblemB(String[] args) {
        if (args.length > 0 && args[0].equals("debug")) {
            debug = true;
        }
    }
    public static void main(String[] args) {
        new ProblemB(args).run();
    }

    private static class FastScanner {
      private BufferedReader in;
      private StringTokenizer tok;
      public FastScanner(InputStream inputstream) {
        in = new BufferedReader(new InputStreamReader(inputstream));
        tok = null;
      }
      private void tokenizeLine() {
        try {
          String line = in.readLine();
          if (line == null) die();
          tok = new StringTokenizer(line);
        } catch (IOException e) {
          die();
        }
      }
      public String next() {
        while (tok == null || !tok.hasMoreTokens()) {
          tokenizeLine();
        }
        return tok.nextToken();
      }
      public boolean hasNextLine() {
        if (tok == null) {
          try {
            tokenizeLine();
          }
          catch (Exception e) {
            return false;
          }
        }
        return true;
      }
      public String nextLine() {
        if (tok == null) {
          tokenizeLine();
        }
        String ret = tok.hasMoreTokens() ? tok.nextToken("\n"): "";
        tok = null;
        return ret;
      }
      public int nextInt() {
        return Integer.parseInt(next());
      }
      public long nextLong() {
        return Long.parseLong(next());
      }
      public double nextDouble() {
        return Double.parseDouble(next());
      }
    }
}