// https://atcoder.jp/contests/abc145/tasks/abc145_c
//
#![allow(unused_imports)]
use std::io::*;
use std::fmt::*;
use std::str::*;
use std::cmp::*;
use std::collections::*;

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };

    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($iter:expr, [ next / $t:tt ]) => {
        {
            let len = read_value!($iter, usize);
            (0..len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
        }
    };

    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_macros)]
macro_rules! dvec {
    ($t:expr ; $len:expr) => {
        vec![$t; $len]
    };

    ($t:expr ; $len:expr, $($rest:expr),*) => {
        vec![dvec!($t; $($rest),*); $len]
    };
}

#[allow(unused_macros)]
macro_rules! ifv {
    ($t:expr, $a:expr, $b: expr) => {
        if $t { $a } else { $b }
    }
}

#[allow(unused_macros)]
macro_rules! fill {
    ($t:expr, $v:expr) => {
        for i in 0..$t.len() {
            $t[i] = $v;
        }
    };
}

#[allow(unused_macros)]
macro_rules! join {
    ($t:expr, $glue:expr) => {
        $t.into_iter().map(|w| w.to_string()).collect::<Vec<_>>().join($glue)
    };
}

#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),*) => {
        println!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
    }
}

fn next_permutation(a: &mut Vec<i64>) -> bool {
    let n = a.len();
    let mut x = n-2;
    while x >= 0 && a[x] >= a[x+1] {
        if x == 0 {
            return false;
        }
        x -= 1;
    }
    let mut y = n-1;
    while y > x && a[y] <= a[x] {
        y -= 1;
    }
    let mut tmp = a[x];
    a[x] = a[y];
    a[y] = tmp;

    // sort
    let mut hp = BinaryHeap::new();
    for i in x+1..n {
        hp.push(a[i]);
    }
    for i in (x+1..n).rev() {
        a[i] = hp.pop().unwrap();
    }
    true
}

const INF: f64 = 1e18;

fn solve(perm: &Vec<i64>, pos: &Vec<(i64, i64)>) -> f64 {
    let mut total = 0.0;
    let n = perm.len();
    for w in 0..n-1 {
        let i = perm[w] as usize;
        let j = perm[w+1] as usize;
        let mut dx2 = (pos[i].0-pos[j].0)*(pos[i].0-pos[j].0);
        let mut dy2 = (pos[i].1-pos[j].1)*(pos[i].1-pos[j].1);
        total += ((dx2+dy2) as f64).sqrt();
    }
    total
}

fn main() {
    input! {
        n: usize,
        pos: [(i64, i64); n]
    };

    let mut perm = vec![0; n];
    for i in 0..n {
        perm[i] = i as i64;
    }

    let mut ans = 0.0;
    let mut cnt = 0.0;
    loop {
        ans += solve(&perm, &pos);
        cnt += 1.0;
        let r = next_permutation(&mut perm);
        if !r {
            break;
        }
    }
    println!("{:.9}", ans / cnt);
}


Measured memory_bytes: 4352