#include <bits/stdc++.h>

typedef long long ll;
typedef long double ld;

using namespace std;

#define rep(i, a, n) for (int i = (a); i < (n); i++)
#define per(i, a, n) for (int i = (n) - 1; i >= (a); i--)

int main(void) {
	ios::sync_with_stdio(false);
	ll n;
	while(cin >> n){
        unordered_map<ll,ll> m1;
        unordered_map<ll,ll> m2;
        int i;
        vector<ll> v1;
        vector<ll> v2;
        ll a,b;
        for(i=0;i<n;i++){
            cin >> a >> b;
            if(m1[a+b]==0) v1.push_back(a+b);
            if(m2[a-b]==0) v2.push_back(a-b);
            m1[a+b]++; m2[a-b]++;
        }
        ll res=0;
        for(i=0;i<v1.size();i++){
            res+=(m1[v1[i]]*(m1[v1[i]]-1));
        }
        for(i=0;i<v2.size();i++){
            res+=(m2[v2[i]]*(m2[v2[i]]-1));
        }
        ll r=n*n;
        cout << setprecision(9) << fixed;
        cout << (double)res/(double)r << endl;
	}

	return 0;
}