import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int bean = sc.nextInt();
		int pack = sc.nextInt();
		int family = sc.nextInt();
		int total = 0;
		for(int i=0;i<family;i++){
			int old = sc.nextInt();
			total += old ;
		}
		sc.close();
		if(bean*pack>=total){
			System.out.println(bean*pack-total);
		}else{
			System.out.println(-1);
		}

	}// mainmethod

} // class