use std::io::stdin;

fn get_line() -> String {
    let mut line = String::new();
    stdin().read_line(&mut line).ok();
    line
}

fn main() {
    let line = get_line();
    let line: Vec<&str> = line.trim().split_whitespace().collect();
    let line: Vec<i32> = line.iter().map(|x| x.parse().unwrap()).collect();

    let k = line[0];
    let n = line[1];

    let mut bean = k * n;

    let line = get_line();
    let line: Vec<&str> = line.trim().split_whitespace().collect();
    let a: Vec<i32> = line.iter().map(|x| x.parse().unwrap()).collect();

    for i in a {
        bean -= i;
    }
    if bean.signum() == 1 {
        println!("{}", bean);
    } else if bean.signum() == 0 {
        println!("{}", 0);
    } else {
        println!("{}", -1);
    }
}