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 B {

	void run() throws IOException {
		double s = (double) ni() / (double) ni(), z = (double) ni() / (double) ni();
		double p = 1;
		double ns = 1 - s;
		double nz = 1 - z;
		double tns = ns, tnz = nz;
		for (int i = 0; i < 100000; i++) {
			p += ns * nz;
			ns *= tns;
			nz *= tnz;
		}
		pw.print(p * s);
	}

	int[] na(int a_len) throws IOException {
		int[] _a = new int[a_len];
		for (int i = 0; i < a_len; i++)
			_a[i] = ni();
		return _a;
	}

	int[][] nm(int a_len, int a_hei) throws IOException {
		int[][] _a = new int[a_len][a_hei];
		for (int i = 0; i < a_len; i++)
			for (int j = 0; j < a_hei; j++)
				_a[i][j] = ni();
		return _a;
	}

	String next() throws IOException {
		while (st == null || !st.hasMoreTokens())
			st = new StringTokenizer(br.readLine());
		return st.nextToken();
	}

	int ni() throws IOException {
		return Integer.parseInt(next());
	}

	long nextLong() throws IOException {
		return Long.parseLong(next());
	}

	String nl() throws IOException {
		return br.readLine();
	}

	void tr(String debug) {
		if (!OJ)
			pw.println(" " + debug);
	}

	static PrintWriter pw;
	static BufferedReader br;
	static StringTokenizer st;
	static boolean OJ;

	public static void main(String[] args) throws IOException {
		long timeout = System.currentTimeMillis();
		OJ = System.getProperty("ONLINE_JUDGE") != null;
		pw = new PrintWriter(System.out);
		br = new BufferedReader(OJ ? new InputStreamReader(System.in) : new FileReader(new File("B.txt")));

		while (br.ready())
			new B().run();
		if (!OJ) {
			pw.println();
			pw.println(System.currentTimeMillis() - timeout);
		}
		br.close();
		pw.close();
	}

}