using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        Console.SetOut(sw);
        Solve();
        Console.Out.Flush();
    }
    static void Solve()
    {
        var N = sc.ReadInt();
        var X = new int[N];
        var Y = new int[N];
        for (int i = 0; i < N; i++)
        {
            X[i] = sc.ReadInt();
            Y[i] = sc.ReadInt();
        }
        var A = Enumerable.Range(0, N).ToArray();
        var sum = 0.0;
        var fac = 0;
        do
        {
            fac++;
            for (int i = 0; i < N - 1; i++)
            {
                var dx = X[A[i]] - X[A[i + 1]];
                var dy = Y[A[i]] - Y[A[i + 1]];
                sum += Math.Sqrt(dx * dx + dy * dy);
            }
        } while (NextPermutation<int>(A));
        var ans = sum / fac;
        Console.WriteLine(ans);
    }

    static bool NextPermutation<T>(T[] a) where T : IComparable
    {
        var n = a.Length;

        // 1: 昇順になっている個所を探す
        int i;
        for (i = n - 2; i >= 0; i--)
        {
            if (a[i].CompareTo(a[i + 1]) < 0) break;
        }
        // 配列がすでに降順であるときは終了
        if (i == -1) return false;

        // 3: a[i]より大きいa[j]を探す(i < j)
        int j;
        for (j = n - 1; j > i; j--)
        {
            if (a[j].CompareTo(a[i]) > 0) break;
        }

        // 4: a[i], a[j] をスワップし、a[i+1]..a[n-1] の範囲をリバース
        var tmp = a[i];
        a[i] = a[j];
        a[j] = tmp;
        Array.Reverse(a, i + 1, n - i - 1);

        return true;
    }


    //static class Permutation
    //{
    //    private static void Search<T>(List<T[]> perms, Stack<T> stack, T[] a)
    //    {
    //        int N = a.Length;
    //        if (N == 0)
    //        {
    //            perms.Add(stack.Reverse().ToArray());
    //        }
    //        else
    //        {
    //            var b = new T[N - 1];
    //            Array.Copy(a, 1, b, 0, N - 1);
    //            for (int i = 0; i < a.Length; ++i)
    //            {
    //                stack.Push(a[i]);
    //                Search(perms, stack, b);
    //                if (i < b.Length) { b[i] = a[i]; }
    //                stack.Pop();
    //            }
    //        }
    //    }
    //    public static List<T[]> All<T>(params T[] source)
    //    {
    //        var perms = new List<T[]>();
    //        Search(perms, new Stack<T>(), source);
    //        return perms;
    //    }
    //}
    static Scanner sc = new Scanner();
}
class Scanner
{
    string[] S = new string[0];
    int Index = 0;
    char[] Separators = new char[] { ' ' };

    public string Next()
    {
        if (this.Index < this.S.Length) return this.S[this.Index++];
        var line = "";
        while (line == "") line = Console.ReadLine();
        this.S = line.Split(this.Separators, StringSplitOptions.RemoveEmptyEntries);
        if (this.S.Length == 0) return this.Next();
        this.Index = 0;
        return this.S[this.Index++];
    }
    public string ReadStr()
    {
        return this.Next();
    }
    public char ReadChar()
    {
        return this.Next()[0];
    }
    public int ReadInt()
    {
        return int.Parse(this.Next());
    }
    public uint ReadUInt()
    {
        return uint.Parse(this.Next());
    }
    public long ReadLong()
    {
        return long.Parse(this.Next());
    }
    public double ReadDouble()
    {
        return double.Parse(this.Next());
    }
    public Tuple<int, int> ReadTup(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add);
    }
    public Tuple<long, long> ReadTupLong(int add = 0)
    {
        return Tuple.Create(this.ReadLong() + add, this.ReadLong() + add);
    }
    public Tuple<int, int, int> ReadTup3(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add);
    }
    public Tuple<int, int, int, int> ReadTup4(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add);
    }
    public int[] ReadIntArray(int n)
    {
        var array = new int[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadInt();
        }
        return array;
    }
    public long[] ReadLongArray(int n)
    {
        var array = new long[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadLong();
        }
        return array;
    }
    public double[] ReadDoubleArray(int n)
    {
        var array = new double[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadDouble();
        }
        return array;
    }
    public char[] ReadCharArray(int n)
    {
        var array = new char[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadChar();
        }
        return array;
    }
    public string[] ReadStrArray(int n)
    {
        var array = new string[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadStr();
        }
        return array;
    }
    public Tuple<long, long>[] ReadTupLongArray(int n, int add = 0)
    {
        var array = new Tuple<long, long>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTupLong(add);
        }
        return array;
    }
    public Tuple<int, int>[] ReadTupArray(int n, int add = 0)
    {
        var array = new Tuple<int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup(add);
        }
        return array;
    }
    public Tuple<int, int, int>[] ReadTup3Array(int n, int add = 0)
    {
        var array = new Tuple<int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup3(add);
        }
        return array;
    }
    public Tuple<int, int, int, int>[] ReadTup4Array(int n, int add = 0)
    {
        var array = new Tuple<int, int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup4(add);
        }
        return array;
    }
}


Measured memory_bytes: 15316