#include <iostream>
#include <map>
#include <set>
#include <algorithm>
#include <limits>
#include <cmath>
#include <cstdio>
#include <vector>
#include <queue>

#define DEBUG

#ifdef DEBUG
	#define DBG if (1)
#else
	#define DBG if (0)
#endif

#define PA(arr, n) DBG for (int i = 0; i < n; i++) printf("%d%c", arr[i], "\n "[i < n - 1])
#define PD(n) DBG printf("%d\n", n); 
#define PDBL(n) DBG printf("%lf\n", n);

using namespace std;

struct coor {
	int x;
	int y;
};

int main () {

	
	int n;
	while(scanf("%d", &n) == 1){

		vector<coor> arr(n);
		//vector<int> count(n,0);
		int count=0;		
		for (int i = 0; i < n; i++) {
			scanf("%d", &arr[i].x);
			scanf("%d", &arr[i].y);	
		}
		
		for(int i = 0; i < n; i++) {
			for(int j = i+1; j < n; j++) {
 				int x = arr[j].x -arr[i].x;
				int y = arr[j].y -arr[i].y;
				if(abs(x) == abs(y))
					count++;			
			}		
		}
		double prob = (double)(count*2)/(double)(n*n);
		printf("%lf\n", prob);
		//PA(arr, n);
	}

	return 0;

}