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

fn read_stdin() -> Vec<String> {
    let mut buffer = String::new();
    io::stdin().read_to_string(&mut buffer).ok();
    buffer.trim().split('\n').map(|s| s.to_string()).collect()
}

fn main() {
    let input = read_stdin();
    let l1 = &input[0].split(' ').map(|s| s.parse::<usize>().unwrap()).collect::<Vec<usize>>();
    let beans = *&l1[0] * *&l1[1];
    let f = &l1[2];
    let ages = *&input[1].split(' ').map(|s| s.parse::<usize>().unwrap()).sum::<usize>();

    if beans < ages {
        println!("{}", -1);
    } else {
        println!("{}", beans - ages);
    }
}