import java.util.*;
import java.util.stream.*;
public class Main{
	public static void main(String... args){
		Scanner sc=new Scanner(System.in);
		int k=sc.nextInt();
		int n=sc.nextInt();
		int f=sc.nextInt();
		int sum=0;
		for(int i=0;i<f;i++){
			sum+=sc.nextInt();
		}
		put(Math.max(-1,k*n-sum));
	}
	
	public static void print(Object object){
        System.out.print(object);
    }
    public static void put(Object object) {
        System.out.println(object);
    }
    public static void put(){
        System.out.println();
    }
 
    public static void print(String format,Object... args){
        System.out.print(String.format(format,args));
    }
    public static void put(String format,Object... args) {
        System.out.println(String.format(format,args));
    }
}