import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
       int k = koko.nextInt();
       int n = koko.nextInt();
       int f = koko.nextInt();
       int[] a = new int[f];
       int rest;
       int all = 0;
       for(int i = 0; i<f; i++){
           a[i] = koko.nextInt();
           all = all + a[i];
       }
       if(k*n>=all){
           rest = k*n-all;
       }else{
           rest = -1;
       }
       System.out.println(rest);
    }
}