use std::io::*;
use std::str::*;

fn read<T: FromStr>(sl: &mut StdinLock) -> Option<T> {
    let s = sl.by_ref().bytes().map(|c| c.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect::<String>();

    s.parse::<T>().ok()
}

fn run(sl: &mut StdinLock) {
    let k = read::<i32>(sl).unwrap();
    let n = read::<i32>(sl).unwrap();
    let f = read::<i32>(sl).unwrap();

    let sum = (0..f).map(|_| read::<i32>(sl).unwrap()).sum();
    let r = if k*n >= sum { k*n - sum } else { -1 };
    println!("{}", r);
}

fn main() {
    let s = stdin();
    let mut sl = s.lock();
    run(&mut sl);
}