#include <iostream>
#include<unordered_map>
#include <iomanip>
#define ll long long
using namespace std;

ll n, ans;
char c;


int main()
{
    ios::sync_with_stdio(false);
    cout << setprecision(10);
   while(cin >> n)
   {
        ll sum = 0;
        unordered_map<int, ll> w;
        unordered_map<int, ll> e;
        for(int i = 1; i <= n; i++) {
            int x, y;
            cin >> x >> y;
            w[x-y]++;
            e[x+y]++;
        }
        for(auto it= w.begin(); it != w.end(); it++) {
            sum += it->second* (it->second-1);
        }
        for(auto it= e.begin(); it != e.end(); it++) {
            sum += it->second* (it->second-1);
        }
        cout << (double)sum / (double)n/(double)n << endl; //TODO
   }
    return 0;
}