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.math.BigInteger;
import java.util.*;

public class B implements Runnable {

    private static final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;

    private BufferedReader in;
    private PrintWriter out;
    private StringTokenizer tok = new StringTokenizer("");

    private void init() throws FileNotFoundException {
        Locale.setDefault(Locale.US);
        String fileName = "";
        if (ONLINE_JUDGE && fileName.isEmpty()) {
            in = new BufferedReader(new InputStreamReader(System.in));
            out = new PrintWriter(System.out);
        } else {
            if (fileName.isEmpty()) {
                in = new BufferedReader(new FileReader("input.txt"));
                out = new PrintWriter("output.txt");
            } else {
                in = new BufferedReader(new FileReader(fileName + ".in"));
                out = new PrintWriter(fileName + ".out");
            }
        }
    }

    String readString() {
        while (!tok.hasMoreTokens()) {
            try {
                tok = new StringTokenizer(in.readLine());
            } catch (Exception e) {
                return null;
            }
        }
        return tok.nextToken();
    }

    int readInt() {
        return Integer.parseInt(readString());
    }

    long readLong() {
        return Long.parseLong(readString());
    }

    double readDouble() {
        return Double.parseDouble(readString());
    }

    int[] readIntArray(int size) {
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = readInt();
        }
        return a;
    }

    public static void main(String[] args) {
        //new Thread(null, new _Solution(), "", 128 * (1L << 20)).start();
        new B().run();
    }

    long timeBegin, timeEnd;

    void time() {
        timeEnd = System.currentTimeMillis();
        System.err.println("Time = " + (timeEnd - timeBegin));
    }

    @Override
    public void run() {
        try {
            timeBegin = System.currentTimeMillis();
            init();
            solve();
            out.close();
            time();
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }

    class Pair {
        final long x, y;

        Pair(long x, long y) {
            this.x = x;
            this.y = y;
        }
    }

    int aiverson(boolean x) {
        return x ? 1 : 0;
    }

    Pair readPair() {
        return new Pair(readInt(), readInt());
    }

    int gcd(int a, int b) {
        return a == 0 ? b : gcd(b % a, a);
    }

    boolean cw(int x1, int y1, int x2, int y2, int x3, int y3) {
        return (x2 - x1) * (y3 - y1) - (x3 - x1) * (y2 - y1) < 0;
    }

    long getDistSqr(int x1, int y1, int x2, int y2) {
        long dx = x1 - x2;
        long dy = y1 - y2;
        return dx * dx + dy * dy;
    }

    private void solve() throws IOException {
        int a = readInt();
        int b = readInt();
        int c = readInt();
        int d = readInt();
        double answer = 1.0 * a / b;
        double curToAdd = 1.0 * a / b;
        while (curToAdd > 1e-11) {
            curToAdd *= (1 - 1.0 * a / b) * (1 - 1.0 * c / d);
            answer += curToAdd;
        }
        out.printf("%.10f", answer);
    }
}