import java.util.*;

public class Main {
    private static Scanner sc = new Scanner(System.in);
    public static void main(String[] args) throws Exception {
        int sum = sc.nextInt()*sc.nextInt();
        int f = sc.nextInt();
        for (int i = 0;i < f;i++) {
            sum -= sc.nextInt();
        }
        if (sum >= 0) {
            System.out.println(sum);
        } else {
            System.out.println(-1);
        }
    }
}