import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int all = sc.nextInt() * sc.nextInt();
        int n = sc.nextInt();
        for (int i = 0; i < n; i++) {
            all -= sc.nextInt();
        }
        System.out.println(Math.max(all, - 1));
    }
}