import java.util.*;
public class Beans {
	public static void main(String arg[]){
		int k,n,f,amount,res,temp=0;
		Scanner sc = new Scanner(System.in);
		k = sc.nextInt();
		n = sc.nextInt();
		f = sc.nextInt();
		amount = k*n;
		for(int i = 0;i<f;i++){
			temp = temp + sc.nextInt();
		}
		res = amount - temp;
		if(res < 0){
			System.out.println(-1);
		}else{
			System.out.println(res);
		}
	}
}