#include <bits/stdc++.h>

using namespace std;

typedef long long int ll;
typedef double ld;

typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<ii> vii;

#define PB push_back
#define FOR(prom, a, b) for ( ll prom = (a); prom < (ll)(b); ++prom )
#define F(a) FOR(i,0,a)
#define FF(a) FOR(j,0,a)

#define EPS (1e-10)

struct Pt{
	ll x,y;
};

int main () {
	ios::sync_with_stdio(false);
	ll N;
	cout<<fixed<<setprecision(8);
	while(cin>>N){
		vector<Pt> a(N);
		F(N)cin>>a[i].x>>a[i].y;
		map<ll,ll> ma,mb;
		F(N){
			ma[a[i].x+a[i].y]++;
			mb[a[i].x-a[i].y]++;
		}
		ll sum=0;
		for(auto & x : ma ) {sum += (x.second)*(x.second-1);}
		for(auto & x : mb ) {sum += (x.second)*(x.second-1);}
		ll total=(N*N);
		cout<<(((ld)sum)/total)<<endl;
	}
	return 0;
}