#include <bits/stdc++.h>

using namespace std;
vector<vector<int>> c;
int n;


int main() {
	int n,x,y;
	ios::sync_with_stdio(false);
	float prob;
	while(scanf("%d", &n) == 1) {
		prob = 0;
		c.clear();
		c.reserve(n);
		for(int i = 0; i < n; i++) {
			vector<int> p(2);
			scanf("%d %d",&x,&y);
			p[0] = x - y;
			p[1] = y + x;
			c.emplace_back(p);
		}
		float count = 0;
		for(int kk = 0; kk < 2; kk++) {
			sort(c.begin(),c.end(), [kk](vector<int> &a, vector<int> &b) {
				return a[kk] < b[kk];
			});
			int xv = c[0][kk];
			float tcount = 0;
			for(auto &cc : c) {
				if(cc[kk] == xv) tcount++;
				else {
					xv = cc[kk];
					count += tcount*(tcount-1)/2;
					tcount = 1;
				}
			}
			count += tcount*(tcount-1)/2;
		}
		
		prob = (float)2*count / (float)(n*n);
		cout << prob << endl;
	}
	return 0;
}