import java.util.Scanner;
  
    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 sumBeans = k*n;
            
            for(int i=0; i<f; i++){
                int a = sc.nextInt();
                sumBeans -= a;
            }
            
            if(sumBeans<0){
                System.out.println(-1);
            }else{
                System.out.println(sumBeans);
                
            }
        }
    }