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

struct Input {
    grain_number: i32,
    bag_number: i32,
    family_members: i32,
    ages: Vec<i32>,
}

fn main() {
    let mut stdin = io::stdin();
    let input = read_input(&mut stdin);

    let n = input.grain_number * input.bag_number;
    let result = input.ages.iter().fold(0, |a, b| a + b);

    let result = n - result;
    if result < 0 {
        print!("-1");
    } else {
        print!("{}", result);
    }
}


fn read_input(stdin: &mut Stdin) -> Input {
    let mut lock = stdin.lock();
    let mut s = String::new();
    lock.read_line(&mut s).expect("can not read first line");
    let tokens: Vec<i32> = s.trim_end().split(" ").map(|s| s.parse().expect("can't parse")).collect();

    s.clear();

    lock.read_line(&mut s).expect("can not read first line");
    let ages: Vec<i32> = s.trim_end().split(" ").map(|s| s.parse().expect("can't parse")).collect();


    Input {
        grain_number: tokens[0],
        bag_number: tokens[1],
        family_members: tokens[2],
        ages,
    }
}