Problem:
Score : 300 points

Problem Statement

There are N towns in a coordinate plane. Town i is located at coordinates (x_i, y_i). The distance between Town i and Town j is \sqrt{\left(x_i-x_j\right)^2+\left(y_i-y_j\right)^2}.

There are N! possible paths to visit all of these towns once. Let the length of a path be the distance covered when we start at the first town in the path, visit the second, third, \dots, towns, and arrive at the last town (assume that we travel in a straight line from a town to another). Compute the average length of these N! paths.

Constraints

2 \leq N \leq 8

-1000 \leq x_i \leq 1000

-1000 \leq y_i \leq 1000

\left(x_i, y_i\right) \neq \left(x_j, y_j\right) (if i \neq j)

(Added 21:12 JST) All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
x_1 y_1
:
x_N y_N

Output

Print the average length of the paths.
Your output will be judges as correct when the absolute difference from the judge's output is at most 10^{-6}.

Sample Input 1

3
0 0
1 0
0 1

Sample Output 1

2.2761423749

There are six paths to visit the towns: 1 → 2 → 3, 1 → 3 → 2, 2 → 1 → 3, 2 → 3 → 1, 3 → 1 → 2, and 3 → 2 → 1.

The length of the path 1 → 2 → 3 is \sqrt{\left(0-1\right)^2+\left(0-0\right)^2} + \sqrt{\left(1-0\right)^2+\left(0-1\right)^2} = 1+\sqrt{2}.

By calculating the lengths of the other paths in this way, we see that the average length of all routes is:

\frac{\left(1+\sqrt{2}\right)+\left(1+\sqrt{2}\right)+\left(2\right)+\left(1+\sqrt{2}\right)+\left(2\right)+\left(1+\sqrt{2}\right)}{6} = 2.276142...

Sample Input 2

2
-879 981
-866 890

Sample Output 2

91.9238815543

There are two paths to visit the towns: 1 → 2 and 2 → 1. These paths have the same length.

Sample Input 3

8
-406 10
512 859
494 362
-955 -475
128 553
-986 -885
763 77
449 310

Sample Output 3

7641.9817824387

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

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);
		}
	}
}