#include <bits/stdc++.h>

using namespace std;

using int64 = long long;

int main()
{
  int N, P[200000];
  cin >> N;
  N *= 2;
  for(int i = 0; i < N; i++) cin >> P[i];
  sort(P, P + N);
  double ret = 0;
  for(int i = 0; i < N; i++) ret += 1.0 * i * P[i];
  cout << fixed << setprecision(15) << ret / (N - 1) << endl;
}