import java.util.Scanner;
class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int mame = sc.nextInt() * sc.nextInt();
		int num = sc.nextInt();
		for(int i = 0 ; i < num ; i++) mame -= sc.nextInt();
		if(mame < 0) {
			System.out.println(-1);
		}else {
			System.out.print(mame);
		}
	}
}