import java.io.*;
import java.util.*;

class Main{
  public static void main(String[] args) {

    Scanner scan=new Scanner(System.in);
    int k=Integer.parseInt(scan.next());
    int n=Integer.parseInt(scan.next());
    int f=Integer.parseInt(scan.next());
    int total=k*n;

    int[] array=new int[f];
    for(int i=0;i<array.length;i++){
      array[i]=Integer.parseInt(scan.next());
    }

    for(int i=0;i<array.length;i++){
      total-=array[i];
      if(total<0){
        System.out.println("-1");
        return;
      }
    }
    
    System.out.println(total);
  }
}