#include <iostream>
#include <sstream>
#include <iomanip>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <algorithm>
#include <numeric>
#include <iterator>
#include <functional>
#include <complex>
#include <queue>
#include <stack>
#include <cmath>
#include <cassert>
using namespace std;
typedef long long LL;
typedef long double LD;
typedef complex<double> CMP;

class TreeUnion { public:
	double expectedCycles(vector <string> tree1, vector <string> tree2, int K)
	{
		vector<int> t1, t2;
		{
			stringstream sin(accumulate(tree1.begin(), tree1.end(), string()));
			for(int v; sin>>v; ) t1.push_back(v);
		}
		{
			stringstream sin(accumulate(tree2.begin(), tree2.end(), string()));
			for(int v; sin>>v; ) t2.push_back(v);
		}

		int N = t1.size() + 1;
		vector<vector<int> > d1 = make_d(t1, N);
		vector<vector<int> > d2 = make_d(t2, N);

		map<int, int> len_to_cnt_2;
		for(int u=0; u<N; ++u)
		for(int v=u+1; v<N; ++v)
			len_to_cnt_2[d2[u][v]] += 2;
		int total = N * (N-1);

		double e = 0.0;
		for(int u=0; u<N; ++u)
		for(int v=u+1; v<N; ++v)
		{
			e += (double)len_to_cnt_2[K - 2 - d1[u][v]] / total;
		}
		return e;
	}

	vector<vector<int> > make_d(const vector<int>& x, int N)
	{
		vector<vector<int> > d(N, vector<int>(N, 0x3fffffff));
		for(int i=0; i<N; ++i) d[i][i] = 0;
		for(int i=0; i<x.size(); ++i) {
			d[i+1][x[i]] = 1;
			d[x[i]][i+1] = 1;
		}
		for(int k=0; k<N; ++k)
		for(int i=0; i<N; ++i)
		for(int j=0; j<N; ++j)
			d[i][j] = min(d[i][j], d[i][k]+d[k][j]);
		return d;
	}
};

// BEGIN CUT HERE
#include <ctime>
double start_time; string timer()
 { ostringstream os; os << " (" << int((clock()-start_time)/CLOCKS_PER_SEC*1000) << " msec)"; return os.str(); }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v)
 { os << "{ ";
   for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
   os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", "); os << " }"; return os; }
void verify_case(const double& Expected, const double& Received) {
 bool ok = (abs(Expected - Received) < 1e-9);
 if(ok) cerr << "PASSED" << timer() << endl;  else { cerr << "FAILED" << timer() << endl;
 cerr << "\to: \"" << Expected << '\"' << endl << "\tx: \"" << Received << '\"' << endl; } }
#define CASE(N) {cerr << "Test Case #" << N << "..." << flush; start_time=clock();
#define END	 verify_case(_, TreeUnion().expectedCycles(tree1, tree2, K));}
int main(){

CASE(0)
	string tree1_[] = {"0"};
	  vector <string> tree1(tree1_, tree1_+sizeof(tree1_)/sizeof(*tree1_)); 
	string tree2_[] = {"0"};
	  vector <string> tree2(tree2_, tree2_+sizeof(tree2_)/sizeof(*tree2_)); 
	int K = 4; 
	double _ = 1.0; 
END
CASE(1)
	string tree1_[] = {"0 1"};
	  vector <string> tree1(tree1_, tree1_+sizeof(tree1_)/sizeof(*tree1_)); 
	string tree2_[] = {"0 1"};
	  vector <string> tree2(tree2_, tree2_+sizeof(tree2_)/sizeof(*tree2_)); 
	int K = 4; 
	double _ = 1.3333333333333333; 
END
CASE(2)
	string tree1_[] = {"0 1"};
	  vector <string> tree1(tree1_, tree1_+sizeof(tree1_)/sizeof(*tree1_)); 
	string tree2_[] = {"0 1"};
	  vector <string> tree2(tree2_, tree2_+sizeof(tree2_)/sizeof(*tree2_)); 
	int K = 6; 
	double _ = 0.3333333333333333; 
END
CASE(3)
	string tree1_[] = {"0 ", "1 1 1"};
	  vector <string> tree1(tree1_, tree1_+sizeof(tree1_)/sizeof(*tree1_)); 
	string tree2_[] = {"0 1 0 ", "1"};
	  vector <string> tree2(tree2_, tree2_+sizeof(tree2_)/sizeof(*tree2_)); 
	int K = 5; 
	double _ = 4.0; 
END
CASE(4)
	string tree1_[] = {"0 1 2 0 1 2 0 1 2 5 6 1", "0 11", " 4"};
	  vector <string> tree1(tree1_, tree1_+sizeof(tree1_)/sizeof(*tree1_)); 
	string tree2_[] = {"0 1 1 0 2 3 4 3 4 6 6", " 10 12 12"};
	  vector <string> tree2(tree2_, tree2_+sizeof(tree2_)/sizeof(*tree2_)); 
	int K = 7; 
	double _ = 13.314285714285713; 
END
/*
CASE(5)
	string tree1_[] = ;
	  vector <string> tree1(tree1_, tree1_+sizeof(tree1_)/sizeof(*tree1_)); 
	string tree2_[] = ;
	  vector <string> tree2(tree2_, tree2_+sizeof(tree2_)/sizeof(*tree2_)); 
	int K = ; 
	double _ = ; 
END
CASE(6)
	string tree1_[] = ;
	  vector <string> tree1(tree1_, tree1_+sizeof(tree1_)/sizeof(*tree1_)); 
	string tree2_[] = ;
	  vector <string> tree2(tree2_, tree2_+sizeof(tree2_)/sizeof(*tree2_)); 
	int K = ; 
	double _ = ; 
END
*/
}
// END CUT HERE
