using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using static System.Console;
using static System.Math;

namespace Atcoder
{
    class Program
    {
        static StreamWriter sw = new StreamWriter(OpenStandardOutput()) { AutoFlush = false };
        static void Main(string[] args)
        {
            try
            {
                SetOut(sw);
                Solve.Answer();
                Out.Flush();
            }
            catch (Exception ex)
            {
                WriteLine(ex.ToString());
            }
        }
    }

    public class Solve
    {
        public static void Answer()
        {
            var n = IO.RInt;
            var xy = new XY[n];
            for (int i = 0; i < n; i++)
            {
                var a = IO.RSInt;
                xy[i] = new XY(a[0], a[1]);
            }

            var range = Enumerable.Range(0, n).ToArray();
            double sum = 0;
            var f = 0;
            foreach (var p in AllPermutation(range))
            {
                f++;

                double res = 0;
                for (int i = 0; i < p.Length - 1; i++)
                {
                    res += Sqrt(Pow(xy[p[i]].X - xy[p[i + 1]].X, 2) + Pow(xy[p[i]].Y - xy[p[i + 1]].Y, 2));
                }
                sum += res;
            }
            var ans = sum / f;
            WriteLine(ans);
        }

        struct XY
        {
            public int X { get; set; }
            public int Y { get; set; }
            public XY(int x, int y)
            {
                this.X = x;
                this.Y = y;
            }
        }

        static List<T[]> AllPermutation<T>(params T[] array) where T : IComparable
        {
            var a = new List<T>(array).ToArray();
            var res = new List<T[]>();
            res.Add(new List<T>(a).ToArray());
            var n = a.Length;
            var next = true;
            while (next)
            {
                next = false;

                int i;
                for (i = n - 2; i >= 0; i--)
                {
                    if (a[i].CompareTo(a[i + 1]) < 0) break;
                }
                if (i < 0) break;

                var j = n;
                do
                {
                    j--;
                } while (a[i].CompareTo(a[j]) > 0);

                if (a[i].CompareTo(a[j]) < 0)
                {
                    var tmp = a[i];
                    a[i] = a[j];
                    a[j] = tmp;
                    Array.Reverse(a, i + 1, n - i - 1);
                    res.Add(new List<T>(a).ToArray());
                    next = true;
                }
            }
            return res;
        }

        public class IO
        {
            #region Input
            public static int RInt => int.Parse(ReadLine());
            public static long RLong => long.Parse(ReadLine());
            public static double RDouble => double.Parse(ReadLine());

            public static string[] RSString => ReadLine().Split();
            public static int[] RSInt => ReadLine().Split().Select(s => int.Parse(s)).ToArray();
            public static long[] RSLong => ReadLine().Split().Select(s => long.Parse(s)).ToArray();
            public static double[] RSDouble => ReadLine().Split().Select(s => double.Parse(s)).ToArray();
            #endregion

            #region Output
            public static void OutJoinSpace<T>(IEnumerable<T> list) => WriteLine(string.Join(" ", list));
            public static void WriteLineYes_or_No(bool flag) => WriteLine(flag ? "Yes" : "No");
            public static void WriteLineYES_or_NO(bool flag) => WriteLine(flag ? "YES" : "NO");
            #endregion
        }

        public class Mod
        {
            #region
            private readonly static long M = 1000000007;

            public static long Add(long a, long b) => (a * b) % M;
            public static long Sub(long a, long b) => (a - b) % M >= 0 ? (a - b) % M : (a - b) % M + M;
            public static long Multi(long a, long b) => (a * b) % M;
            #endregion
        }
    }
}

Measured memory_bytes: 20312