macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

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

macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

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

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

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

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

fn dis(a: &Vec<i64>, b: &Vec<i64>) -> f64 {
    (((a[0]-b[0]).pow(2) + (a[1]-b[1]).pow(2)) as f64).sqrt()
}

fn main(){

    input!{
        n: usize,
        v: [[i64;2];n],
    }
    let mut t:Vec<usize> = (1..n+1).collect();
    let mut sum: f64 = 0.0;
    let mut cnt = 0;
    let mut ans = 0.0;
    if n<2 {
        println!("{}", ans);
        return;
    }
    loop {
        //let mut tmp: Vec<i64> = vec![0,0];
        let mut route = 0.0;
        for i in 1..n+1 {
            if i==n {
                continue;
            } else {
                route += dis(&v[t[i-1]-1], &v[t[i]-1]);
            }
        }
        cnt += 1;
        sum += route;
        if t.next_permutation()==false {
            break;
        }
    }
    ans = sum/(cnt as f64);
    println!("{}", ans);
}

pub trait LexicalPermutation {
    fn next_permutation(&mut self) -> bool;
    fn prev_permutation(&mut self) -> bool;
}

impl<T> LexicalPermutation for [T] where T: Ord {
    fn next_permutation(&mut self) -> bool {
        if self.len() < 2 {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 j >= i && 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() < 2 {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 j >= i && self[j-1] < self[i-1] {
            j -= 1;
        }
        self.swap(i-1, j);
        true
    }

}


Measured memory_bytes: 4352