fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}

fn main() {
    let bean: Vec<i32> = read_vec();
    let membebrs: Vec<i32> = read_vec();
    let total = bean[0] * bean[1];
    let result = total - membebrs.iter().fold(0, |sum, i| sum + i);
    if result >= 0 {
        println!("{}", result);
    } else {
        println!("-1");
    }
}