use std::io::stdin;
pub fn main() {
    let line = || {
        let mut s = String::new();
        let _ = stdin().read_line(&mut s);
        s
    };
    let (k, n, _) = {
        let v: Vec<usize> = line().split_whitespace().flat_map(|x| x.parse()).collect();
        (v[0], v[1], v[2])
    };
    let v: Vec<usize> = line().split_whitespace().flat_map(|x| x.parse()).collect();
    let kn = k * n;
    let sa: usize = v.iter().sum();

    let answer = match (kn as isize) - (sa as isize) {
        x if x < 0 => -1 as isize,
        x => x as isize,
    };
    println!("{}", answer);
}