use proconio::{fastout, input};

fn yes(yes_no: bool) {
    if yes_no {
        println!("Yes");
    } else {
        println!("No");
    }
}

#[fastout]
fn main() {
    input! {
        k: u32,
        n: u32,
        f: u32,
        a: [u32; f],
    }
    let s = a.iter().sum();
    let t = k * n;
    if t >= s {
        println!("{}", t - s);
    } else {
        println!("-1");
    }
}