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

using namespace std;

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

int xy[100001][2];

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

        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;
}