#include <cstdio>
#include <cstring>
#include <vector>
#include <map>
using namespace std;

int main(void){
	for (;!feof(stdin);) {
		int count;
		if (scanf("%d", &count) != 1)
			break;
		vector<vector<int>> coords;
		map<int,int> pravo;
		map<int,int> lavo;
		
		coords.resize(count);
		for ( int i = 0; i < count; i++ ) {
			std::vector<int> inner_coords;
			inner_coords.resize(2);
			int x,y;
			scanf("%d %d", &x, &y);
			inner_coords[0] = x;
			inner_coords[1] = y;
			
			lavo[x+y] += 1;
			pravo[x-y] += 1;
								
			/*else if( x < 0 )
			{
				if( y > 0 ) {
					lavo[x+y] += 1;
					pravo[x-y] += 1;
				}
					
				if( y < 0 ) {
					lavo[x+y] += 1;
					pravo[x-y] += 1;
				}
			}*/
			
			coords[i] = inner_coords;
		}
		int all = count * count;
		int win = 0;
		for ( auto it = pravo.begin(); it != pravo.end(); it++ ) {
			win += it->second * (it->second-1);
		}
		for ( auto it = lavo.begin(); it != lavo.end(); it++ ) {
			win += it->second * (it->second-1);
		}
		
		printf("%f\n", win/double(all));
		//printf("w:%d all:%d  === %f\n", win, all, win/double(all));
	}
	return 0;
}