macro_rules! input {
($($t:ty),*) => {{
let mut s = String::new();
let _ = std::io::stdin().read_line(&mut s);
let mut splits = s.trim().split_whitespace();
($( {splits.next().unwrap().parse::<$t>().unwrap() },)*)}}}

fn main() {
    let knf = input!(usize,usize, usize);
    let mut s = String::new();
    let _ = std::io::stdin().read_line(&mut s);
    let mut splits = s.trim().split_whitespace();

    let mut sum :usize = 0;
    
    for _ in 0..knf.2 {
        sum += splits.next().unwrap().parse::<usize>().unwrap();
    }
    
    if sum>(knf.0*knf.1) {
        println!("-1");
    } else {
        println!("{}", knf.0*knf.1-sum);
    }
}