#include <bits/stdc++.h>
using namespace std;
typedef pair<long long int, long long int> pii;

int main()
{
    int i, N, x, y;
    long long int sum;
    while ( scanf("%d", &N) == 1 )
    {
    	sum=0;
    	map<long long int, pii> m;
    	m.clear();
    	for ( i = 0; i < N; i++ )
    	{
    		scanf("%d%d",&x,&y);
    		m[y+x].first++;
    		m[y-x].second++;
    	}
    	for ( auto & item : m )
    	{
    		sum += (item.second.first*(item.second.first-1) + item.second.second*(item.second.second-1));
    	}
    	printf("%.8Lf\n", (long double)sum/((long long int)N*N));
    }

return 0;
}