// yukicoder Lv.1 No.143 豆
use proconio::input;

fn main() {
    input! {
        k: i32,
        n: i32,
        f: usize,
        a: [i32; f],
    }

    let totalmame = k * n;
    let asum: i32 = a.iter().sum();
    let ans: i32 = if totalmame >= asum {
        totalmame - asum
    } else {
        -1
    };
    println!("{}", ans);
}