#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <utility>
#include <tuple>
#include <algorithm>
#include <numeric>
#include <cstdio>
#include <cmath>
#define rep(i, n) for(int i = 0; i < (n); i++)
using namespace std;
using ll = long long;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

int main() {
  int N;
  cin >> N;
  vector<int> A(N), B(N);
  rep(i, N) cin >> A[i];
  rep(i, N) cin >> B[i];
  vector<int> ai(N), bi(N);
  iota(ai.begin(), ai.end(), 0);
  iota(bi.begin(), bi.end(), 0);
  int res = 0;
  do {
    do {
      vector<int> x(N);
      rep(i, N){
        x[ai[i]] += A[i];
        x[bi[i]] -= B[i];
      }
      int win = 0;
      rep(i, N) {
        if(x[i] > 0)
          win++;
      }
      if(win > N/2.0)
        res++;
    } while(next_permutation(bi.begin(), bi.end()));
  } while(next_permutation(ai.begin(), ai.end()));
  int fac = 1;
  for(int i=2; i<=N; i++)
    fac = fac*i;
  int all = fac*fac;
  cout << (double)res/all << endl;
  return 0;
}