import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Objects;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author silviase
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        CAverageLength solver = new CAverageLength();
        solver.solve(1, in, out);
        out.close();
    }

    static class CAverageLength {
        public void solve(int testNumber, Scanner in, PrintWriter out) {

            int n = in.nextInt();
            Vec2i[] v = new Vec2i[n];
            Integer[] perm = new Integer[n];
            NextPermutation<Integer> np = new NextPermutation<>();
            for (int i = 0; i < n; i++) {
                v[i] = new Vec2i(in.nextInt(), in.nextInt());
                perm[i] = i;
            }

            double ave = 0;
            while (true) {
                for (int i = 1; i < n; i++) ave += Vec2i.dist(v[perm[i]], v[perm[i - 1]]);
                if (np.nextPermutation(perm) == null) break;
            }

            out.println(ave / Arith.factorial(n));


        }

    }

    static class Vec2i {
        public int x;
        public int y;

        public Vec2i(int x, int y) {
            this.x = x;
            this.y = y;
        }

        public String toString() {
            return "Vec2i{" +
                    "x=" + x +
                    ", y=" + y +
                    '}';
        }

        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Vec2i vec2i = (Vec2i) o;
            return x == vec2i.x &&
                    y == vec2i.y;
        }

        public int hashCode() {
            return Objects.hash(x, y);
        }

        public static double dist(Vec2i v1, Vec2i v2) {
            return Math.sqrt(Math.pow(v1.x - v2.x, 2) + Math.pow(v1.y - v2.y, 2));
        }

    }

    static class NextPermutation<T extends Comparable<T>> {
        public T[] nextPermutation(T[] array) {
            int k = -1;
            int i = array.length - 2;
            while (i >= 0) {
                if (array[i].compareTo(array[i + 1]) < 0) {
                    k = i;
                    break;
                }
                i--;
            }

            // 存在しない場合nullを返す
            if (k == -1) return null;
            int l = k + 1;
            i = array.length - 1;
            while (i > k + 1) {
                if (array[k].compareTo(array[i]) < 0) {
                    l = i;
                    break;
                }
                i--;
            }

            T tmp = array[k];
            array[k] = array[l];
            array[l] = tmp;
            int[] res = new int[array.length];
            // k+1 <-> n-1
            for (int j = k + 1; j < array.length + k - j; j++) {
                tmp = array[j];
                array[j] = array[array.length + k - j];
                array[array.length + k - j] = tmp;
            }
            return array;
        }

    }

    static class Arith {
        public static long factorial(int k) {
            return k == 0 ? 1 : factorial(k - 1) * k;
        }

    }
}



Measured memory_bytes: 25652