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

// dfmt off
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;}}
alias sread = () => readln.chomp();enum MOD = 10 ^^ 9 + 7;
alias PQueue = BinaryHeap!(Array!long, "a<b");
// dfmt on

void main()
{
    long N = lread();
    auto XY = new double[2][](N);
    foreach (i; 0 .. N)
        scan(XY[i][0], XY[i][1]);
    auto dist = new double[][](N, N);
    foreach (i; 0 .. N)
        foreach (j; i .. N)
        {
            dist[i][j] = dist[j][i] = sqrt((XY[i][0] - XY[j][0]) ^^ 2 + (XY[i][1] - XY[j][1]) ^^ 2);
        }
    auto ord = iota(N).array();
    double ans = 0;
    long cnt = 0;
    do
    {
        double sum = 0;
        foreach (i; 1 .. N)
        {
            sum += dist[ord[i]][ord[i - 1]];
        }
        ans += sum;
        cnt++;
    }
    while (nextPermutation(ord));
    writefln("%.10f", ans / cnt);
}
