#include <bits/stdc++.h>

using namespace std;

struct node{
public:
	long long int a=0;
	mutable long long  b=0;
	node( long long aa, long long bb):a(aa),b(bb) {
	}
	int operator < (node aa) const{
		return a<aa.a;
	}
};
/*
int binary(long long int slope,	vector < pair < long long , long long > > & pole){
	int lo =0, hi=pole.size()-1,mid;
	while (lo <=hi){
		int mid = lo+hi/2;
		if (slope<pole[mid].first)hi=mid;
		else if (slope>[pole[mid].first) lo=mid;
			else return mid;
	}
	return -1;
}*/


int main () {
	ios::sync_with_stdio(false);
	int n;
	while (cin >> n){	
		set <node> nw,ne;
		vector <pair<long long , long long > >cords;
		for (int iii=0;iii<n;iii++){
			long long x,y;
			cin >> x>>y;
			long long int nee=x+y;
			long long int nww=x-y;
			cords.push_back({nee,nww});
			set<node>::iterator  i=ne.find(node(nee,0)),j=nw.find(node(nww,0));
			if (i==ne.end()) ne.insert(node(nee,1));
				else i->b++;
			if (j==nw.end()) nw.insert(node(nww,1));
				else j->b++;
		}
		long double sum=0;
		for (int ii=0;ii<n;ii++){
			auto i=ne.find(node(cords[ii].first,0));
			auto j=nw.find(node(cords[ii].second,0));
		 	sum+=(i->b + j->b -2)/(n*1.0);
		}
		cout << sum/(n*1.0) << endl;
	}
	return 0;
}