import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        while(sc.hasNext()){
        	int sum=sc.nextInt()*sc.nextInt();
        	int f=sc.nextInt();
        	for(int i=0;i<f;i++) sum-=sc.nextInt();
        	System.out.println(Math.max(sum, -1));
        }
    }
}