#include <stdio.h>
#include <stdlib.h>

typedef struct town {
	int x;
	int y;
} TOWN;

int main() {
	int N;
	int i, j;
	double count;
	while (scanf("%d", &N) > 0) {
		count = 0;
		TOWN *arr = (TOWN*)malloc(N * sizeof(TOWN));
		for (i = 0; i < N; i++) {
			scanf("%d %d", &arr[i].x, &arr[i].y);
			for(j = 0; j < i; j++){
				if(abs((double)(arr[i].x - arr[j].x) / (double)(arr[i].y - arr[j].y)) == 1){
					count ++;
					//printf("%d\n", count);
				}
			}
		}
		double countOfPairs = (double)(N) * (double)(N) / 2;
//		for (i = 0; i < N - 1; i++) {
//			for (j = i + 1; j < N; j++) {
//				if (abs((double)(arr[i].x - arr[j].x) / (double)(arr[i].y - arr[j].y)) == 1)
//					count++;
//			}
//		}
		printf("%g\n", count/countOfPairs);
	}
	return 0;
}