using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Numerics;

namespace AtCoderWorkspace
{
    public class Solver
    {
        public void Solve()
        {
            var cin = new Scanner();
            var n = cin.nextInt();

            var list = new List<Tuple<int, int>>();
            for (int i = 0; i < n; i++)
            {
                list.Add(Tuple.Create(cin.nextInt(),cin.nextInt()));                
            }


            var junretu = Enumerate(list).ToList();

            var ans = 0.0;
            var count = 0;
            foreach (var item in junretu)
            {
                var before = item[0];
                for (int i = 1; i < item.Length; i++)
                {
                    var d = distance(before.Item1,before.Item2, item[i].Item1,item[i].Item2);
                    ans += d;
                    before = item[i];
                }
                count++;
            }
            
            Console.WriteLine(ans/count);
        }

        private double distance(int x1, int y1, int x2, int y2)
        {
            return Math.Sqrt(Math.Pow((x2 - x1), 2) + Math.Pow((y2 - y1), 2));
        }
        public IEnumerable<T[]> Enumerate<T>(IEnumerable<T> items)
        {
            if (items.Count() == 1)
            {
                yield return new T[] { items.First() };
                yield break;
            }
            foreach (var item in items)
            {
                var leftside = new T[] { item };
                var unused = items.Except(leftside);
                foreach (var rightside in Enumerate(unused))
                {
                    yield return leftside.Concat(rightside).ToArray();
                }
            }
        }

    }



    class Scanner
    {
        string[] s;
        int i;

        char[] cs = new char[] { ' ' };

        public Scanner()
        {
            s = new string[0];
            i = 0;
        }

        public string next()
        {
            if (i < s.Length) return s[i++];
            string st = Console.ReadLine();
            while (st == "") st = Console.ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            if (s.Length == 0) return next();
            i = 0;
            return s[i++];
        }

        public int nextInt()
        {
            return int.Parse(next());
        }
        public int[] ArrayInt(int N, int add = 0)
        {
            int[] Array = new int[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextInt() + add;
            }
            return Array;
        }

        public int[] DPArrayInt(int N, int ex)
        {
            int[] Array = new int[N+ex];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextInt();
            }
            return Array;
        }

        public long nextLong()
        {
            return long.Parse(next());
        }

        public long[] ArrayLong(int N, long add = 0)
        {
            long[] Array = new long[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextLong() + add;
            }
            return Array;
        }

        public long[] DPArrayLong(int N, int ex)
        {
            long[] Array = new long[N + ex];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextLong();
            }
            return Array;
        }

        public double nextDouble()
        {
            return double.Parse(next());
        }


        public double[] ArrayDouble(int N, double add = 0)
        {
            double[] Array = new double[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextDouble() + add;
            }
            return Array;
        }
    }   

    class Program
    {
        static void Main(string[] s) => new Solver().Solve();
    }
}


Measured memory_bytes: 27436