#![allow(unused_imports)]
#![allow(non_snake_case)]
use std::cmp::*;
use std::collections::*;
use std::io::Write;

#[allow(unused_macros)]
macro_rules! debug {
    ($($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), std::io::stderr());
            writeln!(err, "{} = {:?}", e, $e).unwrap()
        })*
    };
}

fn infinite_sum(a: f64, r: f64) -> f64 {
    a / (1. - r)
}

fn main() {
    let n = read::<usize>();
    let a = read_vec::<i64>();
    let a_prob = a
        .iter()
        .map(|&x| {
            infinite_sum(
                1.0 / x as f64,
                (1.0 - 1.0 / x as f64) * (1.0 - 1.0 / x as f64),
            )
        })
        .collect::<Vec<_>>();
    debug!(a_prob);
    let asum = a.iter().sum::<i64>();
    let mut dp = vec![vec![vec![0.0; 2]; 1 << n]; 1 << n];
    for state in 0..1 << n {
        let mut kiri_score = 0;
        for j in 0..n {
            if (state >> j) & 1 == 1 {
                kiri_score += a[j];
            }
            if kiri_score >= asum - kiri_score {
                for k in 0..2 {
                    dp[(1 << n) - 1][state][k] = 1.0;
                }
            }
        }
    }

    for s_state in (0..(1 << n) - 1).rev() {
        'p: for p_state in 0..1 << n {
            for j in 0..n {
                if (s_state >> j) & 1 == 0 && (p_state >> j) & 1 == 1 {
                    continue 'p;
                }
            }
            let unselected = (0..n)
                .filter(|&x| (s_state >> x) & 1 == 0)
                .collect::<Vec<_>>();
            // cur senkou
            let mut temp = 0.0;
            for &ai in &unselected {
                let next_s_state = s_state | (1 << ai);
                let next_p_state = p_state | (1 << ai);
                let prob = a_prob[ai];
                temp = f64::max(
                    temp,
                    prob * dp[next_s_state][next_p_state][1]
                        + (1.0 - prob) * dp[next_s_state][p_state][0],
                );
            }
            dp[s_state][p_state][0] = temp;

            let mut temp = 1.0;
            for &ai in &unselected {
                let next_s_state = s_state | (1 << ai);
                let next_p_state = p_state | (1 << ai);
                let prob = a_prob[ai];
                temp = f64::min(
                    temp,
                    (1.0 - prob) * dp[next_s_state][next_p_state][1]
                        + prob * dp[next_s_state][p_state][0],
                );
            }
            dp[s_state][p_state][1] = temp;
        }
    }
    let ans = 1.0 - dp[0][0][1];
    println!("{}", ans);
}

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}