/*
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 */

/* 
 * File:   northwest.cpp
 * Author: cteam025
 *
 * Created on October 28, 2017, 9:42 AM
 */

#include <bits/stdc++.h>

using namespace std;

int main(int argc, char** argv) {
    ios::sync_with_stdio(false);
    int n;
    while (cin >> n) {
        map<int,int> nw, ne;
        
        for (int i = 0; i < n; i++) {
            int x, y;
            cin >> x >> y;
            
            nw[x+y]++;
            ne[x-y]++;
        }
        
        long long sum = 0;
        int tmp;
        for (auto p : nw) {
            tmp = p.second;
            sum += tmp * (tmp-1);
        }
        
        for (auto p : ne) {
            tmp = p.second;
            sum += tmp * (tmp-1);
        }
        
        cout << (sum/(double)(n*n)) << endl;        
    }

    return 0;
}