#include <stdio.h>
#include <stdlib.h>

int cmpfunc(const void * a, const void * b) {
    return (*(int *) a - *(int *) b);
}

int main() {
    int *k_p, *k_m, n, x, y, i, count;
    double ans;
    while (1) {
        if (scanf("%d", &n) != 1) break;
        k_p = malloc(sizeof(int)*n);
        k_m = malloc(sizeof(int)*n);
        for (i = 0; i < n; i++) {
            scanf("%d %d", &x, &y);
            k_p[i] = y - x;
            k_m[i] = y + x;
        }
        qsort(k_p, n, sizeof(int), cmpfunc);
        qsort(k_m, n, sizeof(int), cmpfunc);
        ans = 0;
        count = 1;
        for (i = 1; i < n; i++) {
            if (k_p[i] == k_p[i - 1])
                count++;
            else {
                ans += (double) count/n*(count - 1)/n;
                count = 1;
            }
        }
        ans += (double) count/n*(count - 1)/n;
        count = 1;
        for (i = 1; i < n; i++) {
            if (k_m[i] == k_m[i - 1])
                count++;
            else {
                ans += (double) count/n*(count - 1)/n;
                count = 1;
            }
        }
        ans += (double) count/n*(count - 1)/n;
        printf("%f\n", ans);
    }
    return 0;
}