import java.util.*;

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 beansNumber=k*n;
        for(int i=0;i<f;i++){
            int age=sc.nextInt();
            beansNumber-=age;
        }
        if(beansNumber<0){
            System.out.println(-1);
        }else{
            System.out.println(beansNumber);
        }
    }
}