using System;
using System.Linq;
using System.Collections.Generic;

namespace Contest
{
    class Program
    {
        static void Main(string[] args)
        {
            ABC145.C();
        }



        class ABC145
        {
            public static void A()
            {
                var R = Read.Int();
                Console.WriteLine(R * R);
            }
            public static void B()
            {
                var N = Read.Int();
                var S = Read.Str();
                Console.WriteLine(N % 2 == 0 && S.Substring(0, N / 2) == S.Substring(N / 2, N / 2) ? "Yes" : "No");
            }
            public static void C()
            {
                var N = Read.Int();
                var XY = Read.TupleInt(N);
                double sum = 0;
                for(int i = 0; i < N; ++i)
                {
                    var x1 = XY[i].Item1;
                    var y1 = XY[i].Item2;
                    for (int j = i + 1; j < N; ++j)
                    {
                        var x2 = XY[j].Item1;
                        var y2 = XY[j].Item2;
                        sum += Math.Sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
                    }
                }
                Console.WriteLine(sum / N * 2);
            }
            public static void D()
            {
                var X = Read.Int();
                var Y = Read.Int();
                if ((X + Y) % 3 != 0) { Console.WriteLine(0);return; }
                var a = X - (X + Y) / 3;
                var b = Y - (X + Y) / 3;
                Console.WriteLine(Util.Combin(a + b, 1000000007)(a + b, a));
            }
            public static void E()
            {
                var N = Read.Int();
                var T = Read.Int();
                var AB = Read.TupleInt(N);
                AB = AB.OrderBy(x => x.Item1).ToArray();
                var dp = new int[N + 1, T];
                for(int n = 0; n < N; ++n)
                {
                    var A = AB[n].Item1;
                    var B = AB[n].Item2;
                    for(int t = 0; t < T; ++t)
                    {
                        if (t >= A)
                        {
                            dp[n + 1, t] = Math.Max(dp[n, t], dp[n, t - A] + B);
                        }
                        else
                        {
                            dp[n + 1, t] = dp[n, t];
                        }
                    }
                }
                int max = 0;
                int maxB = 0;
                for(int n = N; n > 0; --n)
                {
                    max = Math.Max(max, dp[n, T - 1] + maxB);
                    maxB = Math.Max(maxB, AB[n - 1].Item2);
                }
                Console.WriteLine(max);
            }
            public static void F()
            {
                var N = Read.Int();
                var K = Read.Int();
                var H = Read.LongN();
                H = new long[] { 0 }.Concat(H).ToArray();
                var dp = new long[N + 1, N - K + 1];
                for (int len = 0; len <= N - K; ++len)
                {
                    dp[0, len] = len == 0 ? 0 : (long)1e18;
                }
                for (int n = 1; n <= N; ++n)
                {
                    dp[n, 0] = (long)1e18;
                    var h = H[n];
                    for(int len = 1; len <= N - K; ++len)
                    {
                        long min = (long)1e18;
                        for(int prev = 0; prev < n; ++prev)
                        {
                            min = Math.Min(min, dp[prev, len - 1] + Math.Max(0, h - H[prev]));
                        }
                        dp[n, len] = min;
                    }
                }
                Console.WriteLine(Enumerable.Range(1, N).Select(n => dp[n, N - K]).Min());
            }
        }


        class Util
        {
            public static long Inverse(long a, long p)
            {
                long b = p, c = 1, d = 0, e = 0, f = 0;
                while (b != 0)
                {
                    e = a / b;
                    f = b; b = a - e * b; a = f;
                    f = d; d = c - e * d; c = f;
                }
                c %= p;
                return c + (c < 0 ? p : 0);
            }

            public static long[] Facts(int n, long p)
            {
                var facts = new long[n + 1];
                facts[0] = 1;
                for (int i = 1; i <= n; ++i)
                {
                    facts[i] = (i * facts[i - 1]) % p;
                }
                return facts;
            }
            public static Func<int, int, long> Combin(int n, long P)
            {
                var fact = Facts(n, P);
                var inv = fact.Select(x => Inverse(x, P)).ToArray();
                return (N, M) =>
                {
                    if (N < 0 || N < M || M < 0) { return 0; }
                    return (((fact[N] * inv[M]) % P) * inv[N - M]) % P;
                };
            }
        }


        static class Read
        {
            static Queue<string> queue = new Queue<string>();
            static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
            public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
            public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
            public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
            public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
            public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
            public static string[] StrN() { return Console.ReadLine().Split(' '); }
            public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
            public static long[,] LongMatrix(int r, int c)
            {
                var mat = new long[r, c];
                for (int i = 0; i < r; ++i)
                {
                    var x = Read.LongN();
                    for (int j = 0; j < c; ++j)
                    {
                        mat[i, j] = x[j];
                    }
                }
                return mat;
            }
            public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M, bool isDirectional = false)
            {
                Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
                for (int i = 0; i <= N; ++i) { dict.Add(i, new HashSet<int>()); }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN(); dict[ab[0]].Add(ab[1]);
                    if (!isDirectional) { dict[ab[1]].Add(ab[0]); }
                }
                return dict;
            }
            public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
            {
                Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new Dictionary<int, long>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1], ab[2]);
                    dict[ab[1]].Add(ab[0], ab[2]);
                }
                return dict;
            }
        }
    }
}


Measured memory_bytes: 13524