import java.util.Scanner;

public class No143 {
 public static void main(String[] args){
	 Scanner sc = new Scanner(System.in);
	 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);
	 }
 }
}