#include <stdio.h>

int arr[10000][4];
int main()
{
	int n;
	int x;
	int y;
	int z;
	int count;

	while(1+scanf("%d", &n))
	{

		x =0;
		y = 0;
		z =0;
		count = 0;
		for(int i=0; i<n; i++)
		{
			scanf("%d %d", &arr[i][0], &arr[i][1]);
			arr[i][2] = 0;
			arr[i][3] = 0;
		}
		for(int i=0; i<n; i++)
		{
			x = 0;
			if(arr[i][2] == 0)
			{
				for(int j=i+1; j<n; j++)
				{
					if(arr[i][0]-arr[j][0] == arr[i][1]-arr[j][1])
					{
						x++;
						arr[j][2] = 1;
					}
				}
				count += x*x+x;
			}


			x = 0;
			if(arr[i][3] == 0)
			{
				for(int j=i+1; j<n; j++)
				{
					if(arr[i][0]-arr[j][0] == -(arr[i][1]-arr[j][1]))
					{
						x++;
						arr[j][3] = 1;
					}
				}
				count += x*x+x;
			}


		}
		printf("%lf\n", (double)count/(n*n));
	}


	return 0;
}