#[macro_export]
macro_rules! setup {
    { mut $input:ident: SplitWhitespace $(,)? } => {
        use std::io::Read;
        let mut buf = String::new();
        std::io::stdin().read_to_string(&mut buf).unwrap();
        let mut $input = buf.split_whitespace();
    };
}

#[macro_export]
macro_rules! parse_next {
    ($str_iter:expr) => {
        $str_iter.next().unwrap().parse().unwrap()
    };
}

fn main() {
    setup! { mut input: SplitWhitespace };

    let k: usize = parse_next!(input);
    let n: usize = parse_next!(input);
    let f: usize = parse_next!(input);
    let a: Vec<usize> = (0..f).map(|_| parse_next!(input)).collect();

    match ((k * n) as i64) - (a.iter().sum::<usize>() as i64) {
        x if x >= 0 => println!("{}", x),
        _ => println!("{}", -1),
    }
}