#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

double memo[1 << 8][9];
int N;
int cards[8];
double rec(int subset, int wins) {
	double &r = memo[subset][wins+4];
	if(r == r) return r;
	if(subset == 0) return wins > 4 ? 1 : 0;
	r = 0;
	int cnt = 0;
	rep(i, N) if(subset >> i & 1)
	reu(j, N, 2*N) if(subset >> j & 1) {
		++ cnt;
		r += rec(subset & ~(1 << i) & ~(1 << j), cards[i] > cards[j] ? wins+1 : wins-1);
	}
	r /= cnt;
	return r;
}

int main() {
	scanf("%d", &N);
	rep(i, 2 * N) scanf("%d", &cards[i]);
	mset(memo, -1);
	double ans = rec((1 << (2 * N)) - 1, 4);
	printf("%.10f\n", ans);
	return 0;
}