#include <stdio.h>
#include <map>
#include <iostream>

using namespace std;

int main()
{
	int N;

	while (scanf("%d", &N) != EOF)
	{
		map<int, int> dias;
		map<int, int> diak;

		long X, Y;

		for (int i = 0; i < N; i++)
		{
			scanf("%ld %ld", &X, &Y);
			if (!diak.count(X - Y))
				diak[X - Y] = 1;
			else
				diak[X - Y] += 1;

			if (!dias.count(X + Y))
				dias[X + Y] = 1;
			else
				dias[X + Y] += 1;
		}

		long long diakSum = 0;
		long long diasSum = 0;

		for (auto it = diak.begin(); it != diak.end(); ++it) {
			int a = it->second;
			diakSum += ((long long)a * (long long)(a - 1));
		}
		for (auto it = dias.begin(); it != dias.end(); ++it) {
			int a = it->second;
			diasSum += ((long long)a * (long long)(a - 1));
		}

		double P = (diakSum + diasSum) / (double)((long long)N * N);
		//printf("%f\n", P);
		cout << P << "\n";

	}

	return 0;
}