fn main() {
    let v = read_vec::<i32>();
    let k = v[0];
    let n = v[1];
    let f = v[2];
    let a = read_vec::<i32>();
    let sum = k * n - a.iter().fold(0,|s,n|s+n);
    println!("{}", if sum < 0 { -1 } else { sum });
}

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>().split_whitespace()
        .map(|e| e.parse().ok().unwrap()).collect()
}

fn read_vec2<T: std::str::FromStr>(n: u32) -> Vec<Vec<T>> {
    (0..n).map(|_| read_vec()).collect()
}