ee

import java.util.ArrayList;

public class DiameterOfRandomTree {

    int n;
    ArrayList<Integer>[] e;
    double[][][] prob;
    boolean[][][] done;


    double calc(int cur, int from, int mustLeq)
    {
        if(mustLeq < 0) return 0;
        if(done[cur][from][mustLeq]) return prob[cur][from][mustLeq];
        double p = 1.0;
        for(int i = 0; i < e[cur].size(); i++)
        {
            int t = e[cur].get(i);
            if(t == from) continue;
            double curProb = 0;
            curProb += 0.5 * calc(t, cur, mustLeq - 1);
            curProb += 0.5 * calc(t, cur, mustLeq - 2);
            p *= curProb;
        }
        done[cur][from][mustLeq] = true;
        prob[cur][from][mustLeq] = p;
        return p;
    }

    double exact(int cur, int from, int l)
    {
        return calc(cur, from, l) - calc(cur, from, l-1);
    }

    public double getExpectation(int[] a, int[] b)
    {
        n = a.length + 1;
        prob = new double[n][n][n+1];
        done = new boolean[n][n][n+1];
        e = new ArrayList[n];
        for(int i = 0; i < n; i++)
            e[i] = new ArrayList<Integer>();
        for(int i = 0; i < a.length; i++)
        {
            e[a[i]].add(b[i]);
            e[b[i]].add(a[i]);
        }
        double ans = 0;
        // center is at middle of edge
        for(int i = 0; i < a.length; i++)
            for(int x = 0; x < n; x ++)
            {
                ans += exact(a[i], b[i], x) * exact(b[i], a[i], x) * (2 * x + 1.5);
                ans += exact(a[i], b[i], x) * exact(b[i], a[i], x-1) * (2 * x + 1) * 0.5;
                ans += exact(a[i], b[i], x-1) * exact(b[i], a[i], x) * (2 * x + 1) * 0.5;
            }

        // center is at vertex
        for(int i = 0; i < n; i++)
            for(int x = 0; x < n; x++)
            {
                double p1 = 1.0;
                for(int j = 0; j < e[i].size(); j++)
                {
                    double curP = 0;
                    curP += 0.5 * calc(e[i].get(j), i, x - 1);
                    curP += 0.5 * calc(e[i].get(j), i, x - 2);
                    p1 *= curP;
                }
                double p2 = 1.0;
                for(int j = 0; j < e[i].size(); j++)
                {
                    double curP = 0;
                    curP += 0.5 * calc(e[i].get(j), i, x - 2);
                    curP += 0.5 * calc(e[i].get(j), i, x - 3);
                    p2 *= curP;
                }
                p1 -= p2;
                for(int s = 0; s < e[i].size(); s++)
                {
                    double p3 = 1.0;
                    for(int j = 0; j < e[i].size(); j++)
                    {
                        if(j != s)
                        {
                            double curP = 0;
                            curP += 0.5 * calc(e[i].get(j), i, x - 2);
                            curP += 0.5 * calc(e[i].get(j), i, x - 3);
                            p3 *= curP;
                        }
                        else
                        {
                            double curP = 0;
                            curP += 0.5 * exact(e[i].get(j), i, x - 1);
                            curP += 0.5 * exact(e[i].get(j), i, x - 2);
                            p3 *= curP;
                        }
                    }
                    p1 -= p3;
                }
                ans += p1 * x * 2;
            }
        return ans;
    }
}