import java.util.*;
         
class Main{
    public static void main(String[] args) {
        
        try(Scanner sc = new Scanner(System.in)){

            int perBz = Integer.parseInt(sc.next());
            int bz = Integer.parseInt(sc.next());
            int fam = Integer.parseInt(sc.next());
            
            int ageSum = 0;
            for(int i = 0; i < fam; i++){
                ageSum += Integer.parseInt(sc.next());
            }

            int ans = perBz * bz - ageSum; 
            if(ans >= 0) System.out.println(ans);
            else System.out.println(-1);

        }catch(Exception e){
            e.printStackTrace();
        }        
    }
}