import java.util.Scanner;
import java.util.Random;

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