import java.util.Scanner;

public class Test {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int s = sc.nextInt() * sc.nextInt(), n = sc.nextInt();
		for (int i = 0; i < n; i++) {
			s -= sc.nextInt();
		}
		if (s < 0) {
			System.out.println(-1);
		} else {
			System.out.println(s);
		}
	}
}