using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCoder.Contest.C
{
	static class Program
	{
		public static void Main(string[] args)
		{
			var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
			Console.SetOut(sw);

			var cin = new Scanner();

			int n = cin.NextInt();
			var p = new List<Pair<int, int>>();
            for (int i = 0; i < n; i++)
            {
				p.Add(new Pair<int, int>(cin.NextInt(), cin.NextInt()));

            }
			var l = new int[n];
            for (int i = 0; i < n; i++)
            {
				l[i] = i;
            }

			double ans = 0;
			double now = 0;
            do
            {
                for (int i = 0; i < n - 1; i++)
                {
					now += GetLength(p[l[i]], p[l[i + 1]]);
                }
				ans += now / Factorial(n);
				now = 0;
            } while (NextPermutation(ref l));

            Console.WriteLine(ans);

			Console.Out.Flush();
		}

		public static long Factorial(long i)
		{
			return i <= 1 ? 1 : i * Factorial(i - 1);
		}

		public static double GetLength(Pair<int, int> a, Pair<int, int> b)
			=> Math.Sqrt((a.First - b.First) * (a.First - b.First) + (a.Second - b.Second) * (a.Second - b.Second));

		public static bool NextPermutation<T>(ref T[] array) where T : IComparable
		{
			int i = array.Length - 2;
			int j = array.Length - 1;
			while (i >= 0 && array[i].CompareTo(array[i + 1]) >= 0) i--;
			if (i < 0) return false;
			T x = array[i];
			while (array[j].CompareTo(x) <= 0) j--;
			Swap(ref array[i], ref array[j]);
			Array.Reverse(array, i + 1, array.Length - i - 1);
			return true;
		}

		public static void Swap<T>(ref T x, ref T y)
		{
			T z;
			z = x;
			x = y;
			y = z;
		}
	}

	public class Pair<T, U> : IComparable, IEquatable<Pair<T, U>> where T : IComparable<T>, IEquatable<T> where U : IComparable<U>, IEquatable<U>
	{
		public T First { get; set; }
		public U Second { get; set; }

		public Pair(T first, U second)
		{
			First = first;
			Second = second;
		}

		public int CompareTo(object obj)
		{
			Pair<T, U> castedObj = (Pair<T, U>)obj;
			int x = First.CompareTo(castedObj.First);
			if (x != 0) return x;
			else return Second.CompareTo(castedObj.Second);
		}

		public static bool operator ==(Pair<T, U> x, Pair<T, U> y) => x.CompareTo(y) == 0;

		public static bool operator !=(Pair<T, U> x, Pair<T, U> y) => x.CompareTo(y) != 0;

		public static bool operator <(Pair<T, U> x, Pair<T, U> y) => x.CompareTo(y) == 1;

		public static bool operator >(Pair<T, U> x, Pair<T, U> y) => x.CompareTo(y) == -1;

		public static bool operator <=(Pair<T, U> x, Pair<T, U> y) => x.CompareTo(y) > 0;

		public static bool operator >=(Pair<T, U> x, Pair<T, U> y) => x.CompareTo(y) < 0;

		public bool Equals(Pair<T, U> x) => First.Equals(x.First) && Second.Equals(x.Second);
	}

	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 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 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;
		}
	}
}

Measured memory_bytes: 15316