#include <bits/stdc++.h>
#include <stdio.h>

using namespace std;

#define DEB(x) cout << #x << " = " << x << endl;

int a[100001];
int b[100001];

int main(void) {
    int n;
    while (scanf("%d", &n) == 1) {
        int x1, x2;
        for (int i = 0; i < n; i++) {
            scanf("%d %d", &x1, &x2);
            a[i] = x1 + x2;
            b[i] = x1 - x2;
        }

        sort(a, a + n);
        sort(b, b + n);

        /*for (int i = 0; i  < n; i++) {
            DEB(a[i])
        }
        for (int i = 0; i  < n; i++) {
            DEB(b[i])
        }*/

        int res = 0;
        int resA = 1;
        int resB = 1;
        for (int i = 0; i < n - 1; i++) {
            if (a[i] == a[i + 1]) resA++;
            else {
                res += resA * (resA - 1);
                resA = 1;
            }
            //DEB(resA)

            if (b[i] == b[i + 1]) resB++;
            else {
                res += resB * (resB - 1);
                resB = 1;
            }
        }
        res += resA * (resA - 1);
        res += resB * (resB - 1);

        double r = res / ((double)n * (double)n);
        printf("%.6lf\n", r);

        /*double all = (double)n * (double)n;
        //cout << all << endl;

        uint64_t cnt = 0;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if (i == j) continue;
                int dx = abs(xy[i][0] - xy[j][0]);
                int dy = abs(xy[i][1] - xy[j][1]);
                if (dx == dy) {
                    cnt++;
                }
            }
        }

        //DEB(cnt);
        double res = cnt / (double)all;*/


        //printf("%.6lf\n", res);
    }

    return 0;
}