use std::io::*;
use std::str::FromStr;
//use std::num;
//use std::cmp;

fn main() {
    let k: usize = read();
    let n: usize = read();
    let f: usize = read();
    let mut a = vec![0; f];
    let mut count = 0;

    for _i in 0..a.len(){
        a[_i] = read();
    }

    for _i in 0..a.len(){
        count += a[_i];
    }

    if k * n >= count{
        println!("{}",k * n - count);
    }else{
        println!("-1");
    }
}

pub fn read_line() -> String {
    let mut s: String = String::new();
    std::io::stdin().read_line(&mut s).ok();

    s
}

pub fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

pub fn read_array() -> Vec<char> {
    let array: String = read();
    let chars: Vec<char> = array.trim().chars().collect::<Vec<char>>();
    chars
}

pub fn get_data() -> Vec<u64> {
    let mut s: String = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let vec: Vec<u64> = s
        .trim()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect();

    vec
}

pub fn get_chars() -> Vec<char> {
    let mut s: String = String::new();
    std::io::stdin().read_line(&mut s).ok();

    let vec: Vec<char> = s.trim().chars().collect::<Vec<char>>();

    vec
}