#include <stdio.h>

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

struct town{
	int x, y;
	int operator - (const town &t) const {
		if (x==t.x && y==t.y) return -1;
		return abs(x-t.x)-abs(y-t.y);
	}
};

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

int main(){
	while(scanf("%d", &n)>=0){
		for(int i=0;i<n;i++){
			scanf("%d%d", &arr[i].x, &arr[i].y);
		}
		w = d = 0;
		
		for(int i=0;i<n;i++){
			for(int j=0;j<n;j++){
				if (arr[i]-arr[j] == 0) w++;
				else d++;
			}
		}
		
		printf("%f\n", (double)w/(double)(w+d));
	}
}