#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for (int i = (a); i <= (b); i++)
#define FORD(i,a,b) for (int i = (a); i >= (b); i--)
#define REP(i,b) for (int i = 0; i < (b); i++)

typedef long long int ll;
typedef long double ld;

const int N = 100000;


int n;
ll x[2*N], y[2*N];

ld res, prob;
ll win, loss, all;
ll cx, cy;

int main() {
    while(scanf("%d", &n) >= 1) {
        REP(i, n) {
            scanf("%lld %lld", &cx, &cy);
            x[i] = cx+cy;
            y[i] = cx-cy;
        }
        sort(x, x+n);
        sort(y, y+n);
        x[n] = 48946846167;
        y[n] = 48946846167;
        win = 0;
        loss =0;
        REP(i, n) {
            int c = 1;
            while(x[i] == x[i+1]) {++i; ++c;}
            win += c*(c-1);
            //printf("w %lld\n",c*(c-1));
        }
        REP(i, n) {
            int c = 1;
            while(y[i] == y[i+1]) {++i; ++c;}
            loss += c*(c-1);
           // printf("l %lld\n",c*(c-1));
        }
        all = n*n;
        prob = (win+loss)/(ld)all;
       // printf("%lld %lld %lld %lld\n", win, loss, all, n);
        printf("%Lf\n", prob);
    }
    return 0;
}