#include <cstdio>
#include <algorithm>
#include <cmath>
#include <chrono>
#include <random>

using namespace std;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

typedef long long ll;
typedef long double ld;

const int N = 1510;
const int RND = 1e6;
const ld OFF = 1e-8;

struct point { 
	ld x, y;
	point() {}
	point(int x, int y) : x(x), y(y) {}
	void rotate() { 
		ld z = y;
		y = -x;
		x = z;
	}

	void print() { 
		printf("%.3LF %.3LF\n", x, y);
	}
};

int n;
point P[N];

point ortho(point a, point b) { 
	if(a.y == b.y) { return point(0, 0); }
	if(a.x > b.x) { swap(a, b); }
	point c = point(b.x - a.x, b.y - a.y);
	c.rotate();
	point d = point((a.x + b.x) / 2, (a.y + b.y) / 2);
	c.x += d.x;
	c.y += d.y;
	return point((c.y - d.y) / (c.x - d.x), c.y - (c.y - d.y) / (c.x - d.x) * c.x);
}

point inter(point a, point b) { 
	point c;
	c.x = (b.y - a.y) / (a.x - b.x);
	c.y = a.x * c.x + a.y;
	return c;
}

ld dist(point a, point b) { 
	return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}

ld abs_(ld x) { return x < 0 ? -x : x; }

int triple(int a, int b, int c) { 
	point l1 = ortho(P[a], P[b]);
	point l2 = ortho(P[a], P[c]);
	if(P[a].y == P[b].y) { l1 = ortho(P[b], P[c]); }
	if(P[a].y == P[c].y) { l2 = ortho(P[b], P[c]); }

	point s = inter(l1, l2);

//	l1.print();
//	l2.print();
//	s.print();

	ld d = dist(s, P[a]);
//	printf("d %.3LF, %.3LF %.3LF %.3LF\n", d, dist(s, P[a]), dist(s, P[b]), dist(s, P[c]));
	int ret = 0;
	for(int i = 0; i < n; ++i) { 
		if(dist(P[i], s) < d || abs_(dist(P[i], s) - d) < OFF) { ++ret; }
	}
//	printf("%d\n", ret);
	return ret;
}

int main() { 
	scanf("%d", &n);
	for(int i = 0; i < n; ++i) { 
		int x, y;
		scanf("%d%d", &x, &y);
		P[i] = point(x, y);
	}

	if(n < 100) { 
		int cnt = 0;
		for(int i = 0; i < n; ++i)
			for(int j = i + 1; j < n; ++j)
				for(int k = j + 1; k < n; ++k) { 
					cnt += triple(i, j, k);
//					printf("%d %d %d %d\n", i, j, k, triple(i, j, k));
				}
		printf("%.3LF\n", (ld) cnt / ((n * (n - 1) * (n - 2)) / 6));
		return 0;
	}

	ll cnt = 0;
	for(int i = 0; i < RND; ++i) { 
		int x = rng() % n;
		int y = rng() % n;
		int z = rng() % n;
		if(x == y || x == z || y == z) { 
			--i;
			continue;
		}

//		int x = rng() % n;
//		int y = rng() % (n - 1);
//		int z = rng() % (n - 2);
//		y += y >= x;
//		if(y > x) { swap(y, x); }
//		z += (z >= y);
//		z += (z >= x);


//		printf("x: "); P[x].print();
//		printf("y: "); P[y].print();
//		printf("z: "); P[z].print();
//		printf("%d %d %d ", x, y, z);
		cnt += triple(x, y, z);
	}
	printf("%.3LF\n", (ld) cnt / RND);
	return 0;
}