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

#[derive(Debug)]
struct Input {
    k: i32,
    n: i32,
    f: i32,
    ages: Vec<i32>,
}

fn next_token(cin_lock: &mut io::StdinLock) -> String {
    cin_lock
        .by_ref()
        .bytes()
        .map(|c| c.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect::<String>()
}

fn read_input(cin_lock: &mut io::StdinLock) -> Input {
    let k = next_token(cin_lock).parse().unwrap();
    let n = next_token(cin_lock).parse().unwrap();
    let f = next_token(cin_lock).parse().unwrap();

    let mut ages = Vec::new();
    for _ in 0..f {
        let age = next_token(cin_lock).parse().unwrap();
        ages.push(age);
    }

    Input {
        k, n, f, ages,
    }
}

fn solve(input: &Input, _cin_lock: &mut io::StdinLock) {
    let required = input.ages.iter().sum();
    let prepared = input.k * input.n;

    let answer = if prepared >= required {
        prepared - required
    }  else {
        -1
    };

    println!("{}", answer);
}

fn main() {
    let cin = io::stdin();
    let mut cin_lock = cin.lock();

    let input = read_input(&mut cin_lock);
    solve(&input, &mut cin_lock);
}