trait NextPermutation {
    fn next_permutation(&mut self) -> bool;
}

impl<T> NextPermutation for Vec<T>
where
    T: PartialOrd,
{
    fn next_permutation(&mut self) -> bool {
        self.windows(2)
            .rposition(|x| x[0] < x[1])
            .map_or(false, |i| {
                let j = self.iter().rposition(|x| x > &self[i]).unwrap();
                self.swap(i, j);
                self[i + 1..].reverse();
                true
            })
    }
}

fn main() {
    let mut xx = String::new();
    std::io::Read::read_to_string(&mut std::io::stdin(), &mut xx).ok();
    let xx: Vec<usize> = xx.split_whitespace().skip(1).flat_map(str::parse).collect();

    let n_cards = xx.len() / 2;
    let mut bb = xx[n_cards..].to_vec();

    bb.sort_unstable();

    let mut n_games = 0.;
    let mut n_a_won = 0.;

    loop {
        n_games += 1.;

        let mut n_a_is_greater = 0;
        for i in 0..n_cards {
            if xx[i] > bb[i] {
                n_a_is_greater += 1;
            }
        }
        if 2 * n_a_is_greater > n_cards {
            n_a_won += 1.;
        }

        if !bb.next_permutation() {
            break;
        }
    }

    println!("{}", n_a_won / n_games);
}