import java.io.*;
class no143
{
 public static void main(String args[])throws IOException
 {
  BufferedReader sc=new BufferedReader(new InputStreamReader(System.in));
  String knf[]=sc.readLine().split(" ");
  String a[]=sc.readLine().split(" ");
  int k,n,f,req=0;
  k=Integer.parseInt(knf[0]);
  n=Integer.parseInt(knf[1]);
  f=Integer.parseInt(knf[2]);
  for(int i=0;i<f;i++)req+=Integer.parseInt(a[i]);
  if(k*n-req>=0)System.out.println(k*n-req);
  else System.out.println(-1);
 }
}