import java.util.Scanner;
class Mame{
  public static void main(String args[]){
    Scanner s = new Scanner(System.in);
    int mame = s.nextInt() * s.nextInt();
    int tabe = 0;
    int F = s.nextInt();
    for(int i = 0; i < F; i++){
      tabe += s.nextInt();
    }
    if(tabe > mame){
      System.out.println("-1");
    }else{
      System.out.println(mame - tabe);
    }
  }
}