using System;
using System.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;

namespace AtCoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            new Program().Solve(new ConsoleInput(Console.In, ' '));
        }

        public void Solve(ConsoleInput cin)
        {
            var N = cin.ReadInt;
            var x = new int[N];
            var y = new int[N];
            for (int i = 0; i < N; i++)
            {
                x[i] = cin.ReadInt;
                y[i] = cin.ReadInt;
            }
            var ans = 0.0;
            Permute(Enumerable.Range(0, N).ToArray(), t =>
            {
                for (int i = 1; i < N; i++)
                {
                    ans += Sqrt(Pow(x[t[i]] - x[t[i - 1]], 2) + Pow(y[t[i]] - y[t[i - 1]], 2));
                }
            });
            for (int i = 1; i <= N; i++)
            {
                ans /= i;
            }
            Console.WriteLine(ans);
        }
        public static void Permute<T>(T[] array, Action<T[]> action)
        {
            Permute(array, 0, array.Length, action);
        }

        private static void Permute<T>(T[] array, int pos, int count, Action<T[]> action)
        {
            if (count == 0)
            {
                action(array);
                return;
            }
            for (var i = 0; i < count; i++)
            {
                Swap(array, pos, pos + i);
                Permute(array, pos + 1, count - 1, action);
                Swap(array, pos, pos + i);
            }
        }

        private static void Swap<T>(T[] array, int x, int y)
        {
            var temp = array[x];
            array[x] = array[y];
            array[y] = temp;
        }
    }

    public class ConsoleInput
    {
        private readonly System.IO.TextReader _stream;
        private char _separator = ' ';
        private Queue<string> inputStream;
        public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
        {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string>();
        }
        public string Read
        {
            get
            {
                if (inputStream.Count != 0) return inputStream.Dequeue();
                string[] tmp = _stream.ReadLine().Split(_separator);
                for (int i = 0; i < tmp.Length; ++i)
                    inputStream.Enqueue(tmp[i]);
                return inputStream.Dequeue();
            }
        }
        public string ReadLine { get { return _stream.ReadLine(); }}
        public int ReadInt { get { return int.Parse(Read); }}
        public long ReadLong { get { return long.Parse(Read); }}
        public double ReadDouble { get { return double.Parse(Read); }}
        public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret;}
        public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret;}
        public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret;}
    }
}


Measured memory_bytes: 13396