use std::io::{self, BufRead};

fn main() {
    let input = io::stdin()
        .lock()
        .lines()
        .flat_map(|l| {
            l.unwrap()
                .split_whitespace()
                .map(|n| n.parse::<i64>().unwrap())
                .collect::<Vec<_>>()
        })
        .collect::<Vec<_>>();

    let n_beans = input[0] * input[1];
    let n_consume = input[3..].iter().sum::<i64>();

    match n_beans - n_consume {
        rem if rem >= 0 => println!("{}", rem),
        _ => println!("-1"),
    }
}