void main()
{
    long n = rdElem;
    Point[] p = n.rdCol!Point;

    double total = 0.0;
    foreach (i; 0 .. n)
    {
        foreach (j; 0 .. n)
        {
            if (i == j) continue;
            total += p[i].dist(p[j]);
        }
    }

    writefln("%.15f", total / n);
}

alias Point = Vector2D!double;

struct Vector2D(T)
{
    T x, y;

    this(T x, T y)
    {
        this.x = x, this.y = y;
    }

    Vector2D!T opBinary(string op, T)(Vector2D!T that)
    if ((op == "+") & isNumeric!T)
    {
        return Vector2D!T(this.x + that.x, this.y + that.y);
    }

    Vector2D!T opBinary(string op, T)(Vector2D!T that)
    if ((op == "-") & isNumeric!T)
    {
        return Vector2D!T(this.x - that.x, this.y - that.y);
    }

    Vector2D!T opBinary(string op, T)(T that)
    if ((op == "*") & isNumeric!T)
    {
        return Vector2D!T(this.x * that, this.y * that);
    }

    Vector2D!T opBinaryRight(string op, T)(T that)
    if ((op == "*") & isNumeric!T)
    {
        return opBinary!(op)(that);
    }

    Vector2D!T opBinary(string op, T)(T that)
    if ((op == "/") & isNumeric!T)
    {
        return Vector2D!T(this.x / that, this.y / that);
    }

    T dot(Vector2D!T that)
    {
        return this.x * that.x + this.y * that.y;
    }

    T cross(Vector2D!T that)
    {
        return this.x * that.y - this.y * that.x;
    }

    T quadrance()
    {
        return dot(this);
    }

    double norm()
    {
        return dot(this).to!double.sqrt;
    }

    double dist(Vector2D!T that)
    {
        return sqrt((this.x - that.x)^^2 + (this.y - that.y)^^2);
    }
}

enum long mod = 10^^9 + 7;
enum long inf = 1L << 60;

T rdElem(T = long)()
if (!is(T == struct))
{
    return readln.chomp.to!T;
}

alias rdStr = rdElem!string;
alias rdDchar = rdElem!(dchar[]);

T rdElem(T)()
if (is(T == struct))
{
    T result;
    string[] input = rdRow!string;
    assert(T.tupleof.length == input.length);
    foreach (i, ref x; result.tupleof)
    {
        x = input[i].to!(typeof(x));
    }
    return result;
}

T[] rdRow(T = long)()
{
    return readln.split.to!(T[]);
}

T[] rdCol(T = long)(long col)
{
    return iota(col).map!(x => rdElem!T).array;
}

T[][] rdMat(T = long)(long col)
{
    return iota(col).map!(x => rdRow!T).array;
}

void rdVals(T...)(ref T data)
{
    string[] input = rdRow!string;
    assert(data.length == input.length);
    foreach (i, ref x; data)
    {
        x = input[i].to!(typeof(x));
    }
}

void wrMat(T = long)(T[][] mat)
{
    foreach (row; mat)
    {
        foreach (j, compo; row)
        {
            compo.write;
            if (j == row.length - 1) writeln;
            else " ".write;
        }
    }
}

import std.stdio;
import std.string;
import std.array;
import std.conv;
import std.algorithm;
import std.range;
import std.math;
import std.numeric;
import std.mathspecial;
import std.traits;
import std.container;
import std.functional;
import std.typecons;
import std.ascii;
import std.uni;
import core.bitop;