#include <bits/stdc++.h>
#define rep(i, n) for (lli i = 0; i < (n); i++)
#define rrep(i, n) for (lli i = (n)-1; i >= 0; i--)
using namespace std;
using lli = long long int;
int main()
{
    double p[200005];
    int n;
    cin >> n;
    rep(i, 2 * n) cin >> p[i];
    sort(p, p + 2 * n, greater<double>());
    double sum = 0;
    rep(i, 2 * n) sum += (2.0 * n - i - 1.0) / (2.0 * n - 1.0) * p[i];
    printf("%.10f\n", sum);
}
