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

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

			List<int> ns = new List<int>();
			for (int i = 0; i < n; i++) {
				ns.Add(i + 1);
			}

			var all = Permutation<int>.All(ns).ToArray();

			double sum = 0;
			for (int i = 0; i < all.Length; i++) {
				var next = all[i];
				double temp = 0;
				for (int j = 1; j < next.Length; j++) {
					int src = next[j - 1] - 1;
					int dst = next[j] - 1;

					temp += Math.Sqrt((xy[dst].Item1 - xy[src].Item1) * (xy[dst].Item1 - xy[src].Item1)
						+ (xy[dst].Item2 - xy[src].Item2) * (xy[dst].Item2 - xy[src].Item2));
				}
				sum += temp;
			}

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

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

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

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

		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: 20820