use std::io::Read;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();
    let mut iter = buf.split_whitespace();
    
    let k: usize = iter.next().unwrap().parse().unwrap();
    let n: usize = iter.next().unwrap().parse().unwrap();
    let mut remaining: isize = k as isize * n as isize;
    let mut f: usize = iter.next().unwrap().parse().unwrap();
    
    while f > 0 {
        let a: isize = iter.next().unwrap().parse().unwrap();
        remaining -= a;
        f -= 1;
    }
    
    println!("{}", if remaining < 0 { -1 } else { remaining });
}