#include <stdio.h>
#include <vector>
#include <cmath>



using namespace std;

vector<int> mesta_x(100066);
vector<int> mesta_y(100066);
vector<int> sum(100066);
		

int main(void){
	
	int N;
	
	
	while(scanf("%d ", &N) == 1){
		double res = 0;
		double total = N*N;
		for(int i = 0; i < N; i++){
			int x, y;
			scanf("%d %d ", &x, &y);
			mesta_x[i] = x;
			mesta_y[i] = y;	
			for(int i1 = i - 1; i1 >= 0; i1--){
				if(abs(mesta_x[i] - mesta_x[i1]) == abs(mesta_y[i] - mesta_y[i1])){
					sum[i] = sum[i1] + 1;
					res += sum[i];
					break;
				}
			}	
		}
		
	printf("%lf\n", (res*2) / total);
	
	}
	
	return 0;
}