#include <iostream>
#include <set>
#include <cstdio>

using namespace std;

struct entry
{
    int val;
    mutable int count;
    entry(int val) : val(val), count(1) { };
};

struct mycomp
{
bool operator()(const entry lhs, const entry rhs)
{
    return lhs.val < rhs.val;
}
};

int main()
{
    int n;
    while (cin >> n)
    {
        set<entry, mycomp> diffset;
	set<entry, mycomp> addset;
	for (int i = 0; i < n; i++)
	{
	    int a, b;
	    cin >> a >> b;
	    set<entry, mycomp>::iterator finddiff, findadd;
	    finddiff = diffset.find(entry(a - b));
	    findadd = addset.find(entry(a + b));
	    if (finddiff == diffset.end())
	    {
	        diffset.insert(entry(a - b));
	    }
	    else
	    {
	        finddiff->count++;
	    }
	    if (findadd == addset.end())
	    {
	        addset.insert(entry(a + b));
	    }
	    else
	    {
	        findadd->count++;
	    }
	}
	double prob = 0;
	for (set<entry, mycomp>::iterator it = diffset.begin(); it != diffset.end(); it++)
	{
	    int k = it->count;
	    prob += (double)k / (double)n * (double)(k - 1) / (double) n;
	}
	for (set<entry, mycomp>::iterator it = addset.begin(); it != addset.end(); it++)
	{
	    int k = it->count;
	    prob += (double)k / (double)n * (double)(k - 1) / (double) n;
	}
	printf("%.9f\n", prob);
    }
    return 0;
}