use std::io::{self, BufReader, BufWriter, Read, Write};

fn main() {
    let stdin = io::stdin();
    let mut stdin = BufReader::new(stdin.lock());
    let stdout = io::stdout();
    let mut stdout = BufWriter::new(stdout.lock());

    let mut buf = String::new();
    stdin.read_to_string(&mut buf).unwrap();

    let mut iter = buf.split_whitespace();
    let k: u32 = iter.next().unwrap().parse().unwrap();
    let n: u32 = iter.next().unwrap().parse().unwrap();
    let f: usize = iter.next().unwrap().parse().unwrap();
    let num = k * n;
    let eat_num: u32 = (0..f)
        .map(|_| iter.next().unwrap().parse::<u32>().unwrap())
        .sum();

    if num < eat_num {
        stdout.write(b"-1").unwrap();
    } else {
        writeln!(&mut stdout, "{}", num - eat_num).unwrap();
    }
}