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

#define DEBUG1

#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-%d%c", arr[i].x, arr[i].y, "\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 {
	long long x;
	long long y;
};

bool cmp1(const coor &a, const coor &b) {
	return a.x+a.y < b.x+b.y;
}

bool cmp2(const coor &a, const coor &b) {
	return a.x-a.y < b.x-b.y;
}

int main () {

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

		vector<coor> arr(n);
		//vector<int> count(n,0);		
		for (int i = 0; i < n; i++) {
			scanf("%d", &arr[i].x);
			scanf("%d", &arr[i].y);	
		}

		sort(arr.begin(), arr.end(), cmp1);
		long long count = 0;
		long long same_count = 0;
		for(int i = 0; i < n-1; i++) {
			if(arr[i].x+arr[i].y == arr[i+1].x+ arr[i+1].y){
				same_count++;
				count += same_count;			
			}
			else {
				same_count = 0;
			}
		}
		//PA(arr,n);
		//PD(count);

		sort(arr.begin(), arr.end(), cmp2);
	
		same_count = 0;		
		for(int i = 0; i < n-1; i++) {
			if(arr[i].x-arr[i].y == arr[i+1].x - arr[i+1].y) {
				same_count++;
				count += same_count;			
			}
			else {
				same_count = 0;
			}
		}
		//PA(arr,n);
		//PD(count);
		/*
		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 = (long double)(count*2)/(long double)(n*n);
		printf("%.6lf\n", prob);
		//PA(arr, n);
	}

	return 0;

}