using System;
using System.IO;
using System.Collections.Generic;
using static System.Console;
using static System.Math;
class Start{
  static void Solve(){
    var sum=Rint()*Rint();
    var n=Rint();
    var eat=0;
    for(int i=0;i<n;i++)eat+=Rint();
    WriteLine(Max(sum-eat,-1));
  }
  static void Main(){
    readcount=-1;
    read=In.ReadToEnd().Split(' ','\n');
    var sw=new StreamWriter(OpenStandardOutput()){AutoFlush=false};
    SetOut(sw);
    Solve();
    Out.Flush();
  }
  const long MOD=1000000007L;
  static void Swap<T>(ref T a,ref T b){T c=a;a=b;b=c;}
  static string dtos(double d,int i){return d.ToString("F"+i);}
  static string Read(){readcount++;return read[readcount];}
  static int Rint(){readcount++;return int.Parse(read[readcount]);}
  static long Rlong(){readcount++;return long.Parse(read[readcount]);}
  static double Rdouble(){readcount++;return double.Parse(read[readcount]);}
  static char Rchar(){readcount++;return read[readcount][0];}
  static string[] read;static int readcount;
}
class Pair<T1,T2>:IComparable<Pair<T1,T2>>where T1:IComparable<T1>where T2:IComparable<T2>{
  public T1 F;public T2 S;
  public Pair(T1 f,T2 s){F=f;S=s;}
  public int CompareTo(Pair<T1,T2> p)=>F.CompareTo(p.F)!=0?F.CompareTo(p.F):S.CompareTo(p.S);
}