import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Northwest {

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        String line;
        while ((line = br.readLine()) != null && !line.isEmpty()) {
            int townsNo = Integer.parseInt(line);
            int[][] towns = new int[townsNo][2];

            for (int i = 0; i < townsNo; i++) {
                StringTokenizer st = new StringTokenizer(br.readLine());
                towns[i][0] = Integer.parseInt(st.nextToken());
                towns[i][1] = Integer.parseInt(st.nextToken());
            }

            double pairs = townsNo * townsNo;
            int winner = 0;

            for (int i = 0; i < townsNo - 1; i++) {
                for (int j = i + 1; j < townsNo; j++) {
                    int[] town1;
                    int[] town2;

                    if (towns[i][1] < towns[j][1]){
                        town1 = towns[i];
                        town2 = towns[j];
                    }else {
                        town1 = towns[j];
                        town2 = towns[i];
                    }

                    double angle = 180*(Math.atan(Math.abs(town1[0] - town2[0])/ (double) Math.abs(town1[1] - town2[1]))/Math.PI);
                    if (angle == 45){
                        winner += 2;
                    }
                }
            }

            System.out.println(winner/pairs);
        }
    }
}