import core.bitop;
import std.algorithm;
import std.ascii;
import std.bigint;
import std.conv;
import std.functional;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.random;
import std.typecons;
import std.container;

ulong MOD = 1_000_000_007;
ulong INF = 1_000_000_000_000;
alias sread = () => readln.chomp();
alias TOWN = Tuple!(long, "x", long, "y");

void main()
{
    auto n = lread();
    auto towns = new TOWN[](n);
    foreach (i; iota(n))
    {
        long x, y;
        scan(x, y);
        towns[i] = TOWN(x, y);
    }
    long cnt;
    double ave_dist = 0;
    foreach (e; towns.permutations())
    {
        cnt++;
        double dist = 0;
        foreach (i; iota(n - 1))
        {
            dist += sqrt(cast(real)(e[i + 1].x - e[i].x) ^^ 2 + (e[i + 1].y - e[i].y) ^^ 2);
        }
        ave_dist += dist;
    }
    writefln("%10.10s",ave_dist / cnt);
}

T lread(T = long)()
{
    return readln.chomp.to!T();
}

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

void scan(TList...)(ref TList Args)
{
    auto line = readln.split();
    foreach (i, T; TList)
    {
        T val = line[i].to!(T);
        Args[i] = val;
    }
}
