/*
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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) {
        if (n == 0) {
            cout << 0 << endl;
            continue;
        }
        
        map<long long, long long> nw, ne;
        
        for (int i = 0; i < n; i++) {
            long long x, y;
            cin >> x >> y;
            
            if (nw.find(x+y) == nw.end()){
                nw.insert(make_pair(x+y, 0));
            }
            
            if (ne.find(x-y) == ne.end()){
                ne.insert(make_pair(x-y, 0));
            }
            
            nw[x+y]++;
            ne[x-y]++;
        }
        
        long long win = 0;
        long long tmp;
        for (auto p : nw) {
            tmp = p.second;
            // cout << p.first << " " << p.second << endl;
            // cout << tmp * (tmp-1) << endl;
            win += tmp * (tmp-1);
        }
        
        for (auto p : ne) {
            tmp = p.second;
            // cout << p.first << " " << p.second << endl;
            // cout << tmp * (tmp-1) << endl;
            win += tmp * (tmp-1);
        }
        
        cout << setprecision(6) << (win/(double)((double)n*n)) << endl;
    }

    return 0;
}