#include <bits/stdc++.h>

#define REP(i, n) for(int i = 0 ; i< (int)n; ++i)

#define X first
#define Y second

using namespace std;

typedef pair<int, int> ii;

bool cmp1(ii a, ii b){
	int sa = a.X + a.Y,
		sb = b.X + b.Y;
	int ma = a.X - a.Y,
		mb = b.X - b.Y;

	if(sa == sb) return ma <= mb;
	return sa <= sb;
}
bool cmp2(ii a, ii b){
	int sa = a.X + a.Y,
		sb = b.X + b.Y;
	int ma = a.X - a.Y,
		mb = b.X - b.Y;

	if(ma == mb) return sa <= sb;
	return ma <= mb;
}

bool testcase(){
	int n;

	if(!(cin >> n))return false;

	long long sm = 0;

	vector<ii> pnts;
	
	int x,y;
	REP(i, n){
		cin >> x >> y;
		pnts.push_back({x,y});
	}

	sort(pnts.begin(), pnts.end(), cmp1);

	

	int lastc = -1;
	long long cnt = 0;

	REP(i, pnts.size()){
		int c = pnts[i].X + pnts[i].Y;
		if(c == lastc) ++cnt;
		else{
			lastc = c;
			sm += (cnt * (cnt-1))/2;
			cnt = 1;
		}
	}
	sm += (cnt * (cnt-1))/2;

	sort(pnts.begin(), pnts.end(), cmp2);

	lastc = -1;
	cnt = 0;

	REP(i, pnts.size()){
		int c = pnts[i].X - pnts[i].Y;
		if(c == lastc) ++cnt;
		else{
			lastc = c;
			sm += (cnt * (cnt-1))/2;
			cnt = 1;
		}
	}
	sm += (cnt * (cnt-1))/2;

	long long total = n*1ll*n;

	cout << setprecision(10) << (2*(double)sm)/((double)total) << endl;

	
	


	return true;
}


int main(){
	while(testcase());
	return 0;
}