Ошибка Access violation at address in modul

#include <vcl.h>
#pragma hdrstop
 
#include "Unit1.h"
#include <math.h>
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
AnsiString a;
AnsiString b;
AnsiString s;
 
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
}
AnsiString DEX_TO_HEX(int N) {
 
AnsiString Hex="0123456789ABCDEF";
    AnsiString H;
 
    do {H.Insert(Hex[N%16+1],1);
            N=N/16;}
    while (N!=0);
    return H;
}
int HEX_TO_DEC(AnsiString st) // ooieoey ia?aaiaa ec 16?e?iie a 10 nenoaio n?eneaiey
{
    int i,s,k,p;
    double j=16;
    s=0;
    char *st1 = st.c_str();
    p=strlen(st1)-1;
    for (i=0; st1[i]!='\0'; i++)
    {
        switch (toupper(st1[i]))
        {
        case 'A': k=10; break;
        case 'B': k=11; break;
        case 'C': k=12; break;
        case 'D': k=13; break;
        case 'E': k=14; break;
        case 'F': k=15; break;
        case '1': k=1; break;
        case '2': k=2; break;
        case '3': k=3; break;
        case '4': k=4; break;
        case '5': k=5; break;
                case '6': k=6; break;
        case '7': k=7; break;
        case '8': k=8; break;
        case '9': k=9; break;
        case '0': k=0; break;
        }
        s=s+k*pow(j,p);
        p--;
    }
    return s;
}
void __fastcall TForm1::ComboBox1Select(TObject *Sender)
{
 
 if (ComboBox1->ItemIndex == 0){ //+
  a = Edit1 -> Text;
  b = Edit2 -> Text;
  int num1 = HEX_TO_DEC(a);
  int num2 = HEX_TO_DEC(b);
  int sum =num1 + num2;
  s = DEX_TO_HEX(sum) ;
  Edit3 -> Text = s; }
 if (ComboBox1->ItemIndex == 1) { //-
  a = Edit1 -> Text;
  b = Edit2 -> Text;
  int num1 = HEX_TO_DEC(a);
  int num2 = HEX_TO_DEC(b);
  int sum =num1 - num2;
  s = DEX_TO_HEX(sum) ;
  Edit3 -> Text = s;}

if (sume<0) sume=~sume+1;