using System;

public class Program
{
    public void Solve()
    {
        var sc = new Scanner();
        int n = sc.NextInt();
        int[] x = new int[n];
        int[] y = new int[n];
        for (int i = 0; i < n; i++)
        {
            x[i] = sc.NextInt();
            y[i] = sc.NextInt();
        }

        int loop = 1;
        for (int i = 0; i < n; i++)
        {
            loop *= n;
        }

        double sum = 0;
        int cnt = 0;
        for (int j = 0; j < loop; j++)
        {
            var p = new int[n]; 
            var b = new bool[n];
            int cp = j;
            bool flag = true;
            for (int i = 0; i < n; i++)
            {
                int d = cp % n;
                if (b[d])
                {
                    flag = false;
                    break;
                }
                b[d] = true;
                p[i] = d;
                cp /= n;
            }
            
            if(!flag) continue; 
            
            for (int i = 1; i < n; i++)
            {
                double xx = x[p[i]] - x[p[i - 1] ];
                double yy = y[p[i] ] - y[p[i - 1] ];
                double dist = Math.Sqrt(xx * xx + yy * yy);
                sum += dist;
            }

            cnt++;
            //Console.WriteLine(string.Join(" ", p));
        } 

        Console.WriteLine(sum / cnt);
    }

    public static void Main(string[] args)
    {
        new Program().Solve();
    }
}

class Scanner
{
    public Scanner()
    {
        _pos = 0;
        _line = new string[0];
    }

    const char Separator = ' ';
    private int _pos;
    private string[] _line;

    #region スペース区切りで取得

    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = Console.ReadLine().Split(Separator);
            _pos = 0;
        }

        return _line[_pos++];
    }

    public int NextInt()
    {
        return int.Parse(Next());
    }

    public long NextLong()
    {
        return long.Parse(Next());
    }

    public double NextDouble()
    {
        return double.Parse(Next());
    }

    #endregion

    #region 型変換

    private int[] ToIntArray(string[] array)
    {
        var result = new int[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = int.Parse(array[i]);
        }

        return result;
    }

    private long[] ToLongArray(string[] array)
    {
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = long.Parse(array[i]);
        }

        return result;
    }

    private double[] ToDoubleArray(string[] array)
    {
        var result = new double[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = double.Parse(array[i]);
        }

        return result;
    }

    #endregion

    #region 配列取得

    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = Console.ReadLine().Split(Separator);

        _pos = _line.Length;
        return _line;
    }

    public int[] IntArray()
    {
        return ToIntArray(Array());
    }

    public long[] LongArray()
    {
        return ToLongArray(Array());
    }

    public double[] DoubleArray()
    {
        return ToDoubleArray(Array());
    }

    #endregion
}

Measured memory_bytes: 15312