#![allow(unused_imports, unused_macros, non_snake_case)]
use std::cmp::{max, min, Ordering};
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};

//{{{ macros missing in 1.15.1
#[macro_use]
mod macros {
    macro_rules! eprint {
        ($($t:tt)*) => {{
            use ::std::io::Write;
            let _ = write!(::std::io::stderr(), $($t)*);
        }};
    }
    macro_rules! eprintln {
        () => { eprintln!(""); };
        ($($t:tt)*) => {{
            use ::std::io::Write;
            let _ = writeln!(::std::io::stderr(), $($t)*);
        }};
    }
    macro_rules! dbg {
        ($val:expr) => {
            match $val {
                tmp => {
                    eprintln!(
                        "[{}:{}] {} = {:#?}",
                        file!(),
                        line!(),
                        stringify!($val),
                        &tmp
                    );
                    tmp
                }
            }
        };
    }
}
//}}}

pub trait Permutation {
    /// It rearranges the elements into the next lexicographically greater permutation.
    ///
    /// Returns `false` without modifying any state only if current is the last permutation.
    fn next_permutation(&mut self) -> bool;

    /// It rearranges the elements into the previous lexicographically greater permutation.
    ///
    /// Returns `false` without modifying any state only if current is the first permutation.
    fn prev_permutation(&mut self) -> bool;
}

impl<T> Permutation for [T]
where
    T: Ord,
{
    fn next_permutation(&mut self) -> bool {
        if self.len() <= 1 {
            return false;
        }

        let mut i = self.len() - 1;
        while i > 0 && self[i - 1] >= self[i] {
            i -= 1;
        }

        if i == 0 {
            return false;
        }

        let mut j = self.len() - 1;
        while self[j] <= self[i - 1] {
            j -= 1;
        }

        self.swap(j, i - 1);

        self[i..].reverse();

        true
    }

    fn prev_permutation(&mut self) -> bool {
        if self.len() <= 1 {
            return false;
        }

        let mut i = self.len() - 1;
        while i > 0 && self[i - 1] <= self[i] {
            i -= 1;
        }

        if i == 0 {
            return false;
        }

        self[i..].reverse();

        let mut j = self.len() - 1;
        while self[j - 1] < self[i - 1] {
            j -= 1;
        }

        self.swap(i - 1, j);

        true
    }
}
fn run() {
    let stdin = std::io::stdin();
    let mut sc = Scanner::new(stdin.lock());
    let n: usize = sc.read();
    let mut ps: Vec<(f64, f64)> = Vec::new();
    for _ in 0..n {
        let x = sc.read();
        let y = sc.read();
        ps.push((x, y));
    }
    let mut perm: Vec<usize> = (0..n).collect::<Vec<_>>();
    let mut count = 0;
    let mut sum = 0.0;
    loop {
        let mut total = 0.0;
        for i in 0..n - 1 {
            let (x1, y1) = ps[perm[i]];
            let (x2, y2) = ps[perm[i + 1]];
            let d = (x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2);
            let d = d.sqrt();
            total += d;
        }
        count += 1;
        sum += total;
        if !perm.next_permutation() {
            break;
        }
    }
    println!("{}", sum / count as f64);
}

fn main() {
    std::thread::Builder::new()
        .name("run".to_string())
        .stack_size(1024 * 1024 * 1024)
        .spawn(run)
        .unwrap()
        .join()
        .unwrap()
}

//{{{ Scanner
pub struct Scanner<R> {
    reader: R,
}

impl<R> Scanner<R> {
    pub fn new(r: R) -> Scanner<R> {
        Scanner { reader: r }
    }
}

fn is_whitespace(b: u8) -> bool {
    b == b' ' || b == b'\n' || b == b'\r' || b == b'\t'
}

impl<R: std::io::Read> Scanner<R> {
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        let buf = std::io::Read::bytes(self.reader.by_ref())
            .map(|b| b.expect("Read failed"))
            .skip_while(|&b| is_whitespace(b))
            .take_while(|&b| !is_whitespace(b))
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error")
    }

    pub fn read_vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| self.read()).collect()
    }

    pub fn read_chars(&mut self) -> Vec<char> {
        self.read::<String>().chars().collect()
    }
}
//}}}


Measured memory_bytes: 8572