class Family {
	int age;

	Family(int i) {
		this.age = i;
	}
}

public class No_143 {

	public static void main(String[] args) {
		java.util.Scanner sc = new java.util.Scanner(System.in);
		int beans = sc.nextInt() * sc.nextInt();
		Family[] yuki = new Family[sc.nextInt()];
		boolean b = true;

		for (int i = 0; i < yuki.length; i++) {
			yuki[i] = new Family(sc.nextInt());
			beans -= yuki[i].age;
			if (beans < 0) {
				b = false;
				break;
			}
		}

		if (b) {
			System.out.println(beans);
		} else {
			System.out.println(-1);
		}
		sc.close();

	}

}