package yukicoder143;
import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		int k=sc.nextInt();//一袋K粒
		int n=sc.nextInt();//N袋
		int f=sc.nextInt();//家族人数
		int sum=0;
		for(int i=0;i<f;i++){
			sum+=sc.nextInt();
		}
		if(k*n>=sum){
			System.out.println(k*n-sum);
		}else{
			System.out.println(-1);
		}
	}
}