import java.util.*;

public class Euler938 {
    public static double solve(int R, int B) {
        double[] prev = new double[R + 1];
        double[] curr = new double[R + 1];

        for (int b = 1; b <= B; ++b) {
            curr[0] = 1.0;
            for (int r = 1; r <= R; ++r) {
                if (r == 1) {
                    curr[r] = prev[r];
                    continue;
                }
                double numerator = (r - 1) * curr[r - 2] + (2 * b) * prev[r];
                double denominator = r - 1 + 2 * b;
                curr[r] = numerator / denominator;
            }
            double[] temp = prev;
            prev = curr;
            curr = temp;
        }

        return prev[R];
    }

    public static String getAnswer() {
        return String.format(Locale.US, "%.10f", solve(24690, 12345));
    }

    public static void main(String[] args) {
        System.out.println(getAnswer());
    }
}