#include <bits/stdc++.h>
#define xx first
#define yy second
#define mp make_pair
#define pb push_back
#define mset(x, y) memset(x, y, sizeof x)
#define mcpy(x, y) memcpy(x, y, sizeof x)
using namespace std;

typedef long long LL;
typedef pair <int, int> pii;

inline int Read()
{
	int x = 0, f = 1, c = getchar();
	for (; !isdigit(c); c = getchar())
		if (c == '-')
			f = -1;
	for (;  isdigit(c); c = getchar())
		x = x * 10 + c - '0';
	return x * f;
}

const int MAXN = 200005;

int n, a[MAXN];

int main()
{
#ifdef wxh010910
	freopen("data.in", "r", stdin);
#endif
	n = Read();
	for (int i = 0; i < n << 1; i ++)
		a[i] = Read();
	sort(a, a + (n << 1));
	LL ans = 0;
	for (int i = 0; i < n << 1; i ++)
		ans += 1LL * a[i] * i;
	return printf("%.9lf\n", (double)ans / ((n << 1) - 1)), 0;
}