using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;

namespace AtCorder
{
	public class Program
	{
		static void Main(string[] args)
		{
			var cin = new Scanner2();
			int n = cin.Int();
			var xy = new Tuple<int, int>[n];
			for (int i = 0; i < n; i++) {
				int x = cin.Int();
				int y = cin.Int();
				xy[i] = new Tuple<int, int>(x, y);
			}

			long cnt = 0;
			double sum = 0;
			var pattern = Permutation.All(n);
			foreach (var p in pattern) {
				++cnt;
				double d = 0;
				for (int i = 1; i < n; i++) {
					var t1 = xy[p[i] - 1];
					var t2 = xy[p[i - 1] - 1];

					d += Math.Sqrt(
						(t1.Item1 - t2.Item1) * (t1.Item1 - t2.Item1) 
						+ (t1.Item2 - t2.Item2) * (t1.Item2 - t2.Item2));
				}

				sum += d;
			}

			double ans = sum / cnt;
			Console.WriteLine(ans);
		}
	}

	static class Permutation
	{
		public static IReadOnlyList<T[]> All<T>(IEnumerable<T> src)
		{
			var perms = new List<T[]>();
			Search(perms, new Stack<T>(), src.ToArray());
			return perms;
		}

		public static IReadOnlyList<int[]> All(int n)
		{
			return All<int>(Enumerable.Range(1, n));
		}

		public static bool Next<T>(T[] src)
			where T : IComparable<T>
		{
			return Next(src, 0, src.Length);
		}

		public static bool Next<T>(T[] src, int index, int length)
			where T : IComparable<T>
		{
			if (length <= 1) {
				return false;
			}

			int last = index + length - 1;
			int i = last;

			while (true) {
				int ii = i;
				--i;
				if (src[i].CompareTo(src[ii]) < 0) {
					int j = last;
					while (src[i].CompareTo(src[j]) >= 0) {
						--j;
					}

					T temp = src[i];
					src[i] = src[j];
					src[j] = temp;
					Array.Reverse(src, ii, last - ii + 1);
					return true;
				}

				if (i == index) {
					Array.Reverse(src, index, length);
					return false;
				}
			}
		}

		public static bool Prev<T>(T[] src)
			where T : IComparable<T>
		{
			return Prev(src, 0, src.Length);
		}

		public static bool Prev<T>(T[] src, int index, int length)
			where T : IComparable<T>
		{
			if (length <= 1) {
				return false;
			}

			int last = index + length - 1;
			int i = last;

			while (true) {
				int ii = i;
				--i;
				if (src[ii].CompareTo(src[i]) < 0) {
					int j = last;
					while (src[j].CompareTo(src[i]) >= 0) {
						--j;
					}

					T temp = src[i];
					src[i] = src[j];
					src[j] = temp;
					Array.Reverse(src, ii, last - ii + 1);
					return true;
				}

				if (i == index) {
					Array.Reverse(src, index, length);
					return false;
				}
			}
		}

		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 class Scanner2
	{
		private readonly char[] delimiter_ = new char[] { ' ' };
		private readonly string filePath_;
		private readonly Func<string> reader_;
		private string[] buf_;
		private int index_;

		public Scanner2(string file = "")
		{
			if (string.IsNullOrWhiteSpace(file)) {
				reader_ = Console.ReadLine;
			} else {
				filePath_ = file;
				var fs = new StreamReader(file);
				reader_ = fs.ReadLine;
			}
			buf_ = new string[0];
			index_ = 0;
		}

		public string Next()
		{
			if (index_ < buf_.Length) {
				return buf_[index_++];
			}

			string st = reader_();
			while (st == "") {
				st = reader_();
			}

			buf_ = st.Split(delimiter_, StringSplitOptions.RemoveEmptyEntries);
			if (buf_.Length == 0) {
				return Next();
			}

			index_ = 0;
			return buf_[index_++];
		}

		public int Int() => int.Parse(Next());
		public long Long() => long.Parse(Next());
		public double Double() => double.Parse(Next());

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

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

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

		public void Save(string text)
		{
			if (string.IsNullOrWhiteSpace(filePath_)) {
				return;
			}

			File.WriteAllText(filePath_ + "_output.txt", text);
		}
	}
}

Measured memory_bytes: 22612