public class Euler978 {
    public static String solve() {
        long a0 = 0, a1 = 1, c0 = 0, c1 = 1;
        int n = 50;
        for (int i = 2; i <= n; i++) {
            long aNew = a1 + a0;
            long cNew = c1 + 3 * c0;
            a0 = a1;
            a1 = aNew;
            c0 = c1;
            c1 = cNew;
        }
        double var = a1 - 1.0;
        if (var <= 0)
            return "0.00000000";
        double m3 = c1 - 3.0 * a1 + 2.0;
        double sigma = Math.sqrt(var);
        double skewness = m3 / (sigma * sigma * sigma);
        return String.format("%.8f", skewness);
    }

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