import java.util.*;
class C{
	static Scanner s = new Scanner(System.in);
	public static void main(String[] args){
		int in=s.nextInt()*s.nextInt();
		s.next();
		while(s.hasNext()) {
			in-=s.nextInt();
		}
		if(in>=0)
			System.out.println(in);
		else
			System.out.println(-1);
	}
}