#include <stdio.h>
#include <algorithm>

int abs(int a){
	return a<0?-a:a;
}

struct town{
	int x, y;
};

bool comp1(const town &a, const town &b) {
	return (a.x-a.y) < (b.x-b.y);
}

bool comp2(const town &a, const town &b) {
	return (a.x+a.y) < (b.x+b.y);
}

int n;
town arr[200000];
int w;


int main(){
	while(scanf("%d", &n)>=0){
		for(int i=0;i<n;i++){
			scanf("%d%d", &arr[i].x, &arr[i].y);
		}
		if (n==1) {
			printf("0\n");
			return 0;
		}
		
		w = 0;
		
		std::sort(arr, arr+n, comp1);
		int c = 1; 
		for(int i=1; i<n;i++){
			if ((arr[i].x-arr[i].y) == (arr[i-1].x-arr[i-1].y)) c++;
			else {
				w += c*c-c;
				//printf("%d\n", w);
				c = 1;
			}
		}
		w += c*c-c;
		std::sort(arr, arr+n, comp2);
		c = 1; 
		for(int i=1; i<n;i++){
			if ((arr[i].x+arr[i].y) == (arr[i-1].x+arr[i-1].y)) c++;
			else {
				w += c*c-c;\
				//printf("%d\n", w);
				c = 1;
			}
		}
		w += c*c-c;
		
		printf("%f\n", (double)w/(double)(n*n));
	}
}