using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var map = NArr(n);

        var p = 0L;
        var q = 0L;
        var r = (long)n * (n - 1) / 2;
        var s = r;

        var xset = new HashSet<int>();
        var yset = new HashSet<int>();
        var rdic = new Dictionary<int, int>();
        var sdic = new Dictionary<int, int>();
        foreach (var pt in map)
        {
            xset.Add(pt[0]);
            yset.Add(pt[1]);
            if (rdic.ContainsKey(pt[0])) ++rdic[pt[0]];
            else rdic[pt[0]] = 1;
            if (sdic.ContainsKey(pt[1])) ++sdic[pt[1]];
            else sdic[pt[1]] = 1;
        }
        foreach (var ri in rdic) r -= (long)ri.Value * (ri.Value - 1) / 2;
        foreach (var si in sdic) s -= (long)si.Value * (si.Value - 1) / 2;
        var xlist = new List<int>(xset);
        xlist.Sort();
        var ylist = new List<int>(yset);
        ylist.Sort();
        var xdic = new Dictionary<int, int>();
        for (var i = 0; i < xlist.Count; ++i) xdic[xlist[i]] = i;
        var ydic = new Dictionary<int, int>();
        for (var i = 0; i < ylist.Count; ++i) ydic[ylist[i]] = i;

        var plist = new List<int>[ylist.Count];
        for (var i = 0; i < plist.Length; ++i) plist[i] = new List<int>();
        for (var i = 0; i < n; ++i)
        {
            plist[ydic[map[i][1]]].Add(xdic[map[i][0]]);
        }
        var pft = new FenwickTree(xlist.Count);
        var mft = new FenwickTree(xlist.Count);
        for (var i = 0; i < plist.Length; ++i)
        {
            foreach (var x in plist[i]) pft.Add(x, 1);
        }
        for (var i = 0; i < plist.Length; ++i)
        {
            foreach (var x in plist[i]) pft.Add(x, -1);
            foreach (var x in plist[i])
            {
                p += pft.Sum(xlist.Count - 1) - pft.Sum(x);
                if (x > 0) p += mft.Sum(x - 1);
                q += mft.Sum(xlist.Count - 1) - mft.Sum(x);
                if (x > 0) q += pft.Sum(x - 1);
            }
        }
        WriteLine((p - q) / Math.Sqrt(r) / Math.Sqrt(s));
    }
    class FenwickTree
    {
        int size;
        long[] tree;
        public FenwickTree(int size)
        {
            this.size = size;
            tree = new long[size + 2];
        }
        public void Add(int index, int value)
        {
            ++index;
            for (var x = index; x <= size; x += (x & -x)) tree[x] += value;
        }
        /// <summary>先頭からindexまでの和(include index)</summary>
        public long Sum(int index)
        {
            if (index < 0) return 0;
            ++index;
            var sum = 0L;
            for (var x = index; x > 0; x -= (x & -x)) sum += tree[x];
            return sum;
        }
        public long Get(int index)
        {
            if (index == 0) return Sum(0);
            return Sum(index) - Sum(index - 1);
        }
        /// <summary>Sum(x) >= value となる最小のxを求める</summary>
        // 各要素は非負であること
        public int LowerBound(long value)
        {
            if (value < 0) return -1;
            var x = 0;
            var b = 1;
            while (b * 2 <= size) b <<= 1;
            for (var k = b; k > 0; k >>= 1)
            {
                if (x + k <= size && tree[x + k] < value)
                {
                    value -= tree[x + k];
                    x += k;
                }
            }
            return x;
        }
    }
}