fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let n: usize = n.trim().parse().unwrap();
    let mut a = String::new();
    std::io::stdin().read_line(&mut a).ok();
    let a: Vec<usize> = a.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();

    println!("{:.09}", 1000. - 1000. * a.iter().map(|&v| (1000. - v as f64) / 1000.).fold(1., |x, y| x * y));
}