#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>

using namespace std;

const int N = 60;

int n;
vector<int> g[N];
double dp[N][2*N][2*N];
int used[N];
int sz[N];
double ndp[2*N][2*N];

void dfs(int v)
{
	used[v]=1;
	vector<int> sons;
	sz[v]=1;

	for (int i=0;i<g[v].size();i++)
	{
		int to=g[v][i];
		if (used[to])
			continue;
		dfs(to);
		sons.push_back(to);
	}

	for (int i=0;i<=sz[v]*2;i++)
		for (int j=0;j<=sz[v]*2;j++)
			dp[v][i][j]=0;

	dp[v][0][0]=1;

	sz[v]=1;

	for (int ii=0;ii<sons.size();ii++)
	{
		int to=sons[ii];
		for (int i=0;i<=(sz[v]+sz[to])*2;i++)
			for (int j=0;j<=(sz[v]+sz[to])*2;j++)
				ndp[i][j]=0;

		for (int i=0;i<=sz[v]*2;i++)
			for (int j=0;j<=sz[v]*2;j++)
				if (dp[v][i][j]>1e-20)
					for (int q=0;q<=sz[to]*2;q++)
						for (int w=0;w<=sz[to]*2;w++)
						{
							ndp[max(i,q+1)][max(j,max(w,q+1+i))]+=dp[v][i][j]*dp[to][q][w]*.5;
							ndp[max(i,q+2)][max(j,max(w,q+2+i))]+=dp[v][i][j]*dp[to][q][w]*.5;
						}

		sz[v]+=sz[to];
		for (int i=0;i<=sz[v]*2;i++)
			for (int j=0;j<=sz[v]*2;j++)
				dp[v][i][j]=ndp[i][j];
	}
}

class DiameterOfRandomTree {
public:
	double getExpectation(vector <int> a, vector <int> b) {

		int n=a.size()+1;
		for (int i=0;i<=n;i++)
			g[i].clear();

		for (int i=0;i<=n;i++)
			used[i]=0;

		for (int i=0;i<a.size();i++)
		{
			g[a[i]].push_back(b[i]);
			g[b[i]].push_back(a[i]);
		}

		dfs(0);

		double ans=0;

		//cout<<sz[0]<<endl;

		for (int i=0;i<=2*sz[0];i++)
			for (int j=0;j<=2*sz[0];j++)
			{
				ans+=dp[0][i][j]*max(i,j);
			//	if (n<6)
			//		cout<<i<<"%"<<j<<" "<<dp[0][i][j]<<" "<<dp[0][i][j]*max(i,j)<<" "<<ans<<endl;

			}

		return ans;
	}
};