fn main() {
    let mut n = String::new();
    std::io::stdin().read_line(&mut n).ok();
    let n: usize = n.trim().parse().unwrap();
    let mut x = String::new();
    std::io::stdin().read_line(&mut x).ok();
    let x: Vec<isize> = x.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let mut m = String::new();
    std::io::stdin().read_line(&mut m).ok();
    let m: usize = m.trim().parse::<usize>().unwrap() - 1;

    let summary: isize = x.iter().sum();
    let avg: f64 = summary as f64 / n as f64;
    let diff2 = (x.iter().map(|&i| (i as f64 - avg) * (i as f64 - avg)).sum::<f64>() / (n as f64)).sqrt();
    let result: f64 = if summary == x[m] * n as isize {
            50.0
        } else {
            50.0 - (avg - (x[m] as f64)) * 10.0 / diff2
        }
    ;
    println!("{}", result.abs().floor() as isize * if result < 0.0 { -1isize } else { 1isize });
}