import std.algorithm;
import std.array;
import std.container;
import std.conv;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.typecons;

void scan(T...)(ref T a) {
    string[] ss = readln.split;
    foreach (i, t; T) a[i] = ss[i].to!t;
}
T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;

double calc(int[][] a) {
    a.sort();
    double ans = 0;
    int n = 0;
    foreach (e; permutations(a)) {
        for (int i = 1; i < e.length; i++) {
            double x1 = e[i - 1][0];
            double y1 = e[i - 1][1];
            double x2 = e[i][0];
            double y2 = e[i][1];
            ans += sqrt((x1 - x2) ^^ 2 + (y1 - y2) ^^ 2);
        }
        n++;
    }
    return ans / n;
}

void main() {
    int n = readint;
    int[][] a;
    foreach (_; 0..n) {
        a ~= readints;
    }

    auto ans = calc(a);
    writefln("%.8f", ans);
}
