use std::io::*;
use std::str::FromStr;
use std::cmp;

#[allow(dead_code)]
fn get_line() -> String {
    let stdin = stdin();
    let mut line = String::new();
    stdin.lock().read_line(&mut line).expect("io error.");
    line.trim().to_string()
}

#[allow(dead_code)]
fn get_word<T: FromStr>() -> T {
    (&get_line()).parse().ok().expect("parse error.")
}

#[allow(dead_code)]
fn get_vec<T: FromStr>() -> Vec<T> {
    (&get_line()).split(' ').map(|x| x.parse().ok().expect("parse error.")).collect()
}

fn main() {
    let v = get_vec::<i64>();
    let a = get_vec::<i64>();

    let ans = v[0] * v[1] - a.iter().sum::<i64>();
    println!("{}", cmp::max(ans, -1));
}