Passing information between textbox and program source code

So I have myself a project with two files in it, AT.c and main.c (I use DEV-C++). AT.c contains all the calculation stuff I need, uses command line for I/O and works relatively fine by itself. However, since I need a GUI for the program, I also got the main.c file, which basically creates a window with textboxes for me to put the numbers in.

Anyway, now I'm kinda stuck because I have no idea how to make AT.c read values from those textboxes instead of using the scanf method. I suppose I should probably make some declarations or links between the two codes, but I haven't found anything that could help yet.

The code is main.c is the starting one you get when creating a new windows application project in Dev-C++. I only altered the LRESULTCALLBACK part.

#define ID_BUTTON 1
#define ID_TEXTBOX 2

static HWND hwndA;
static HWND hwndB;
/*  This function is called by the Windows function DispatchMessage()  */


LRESULT CALLBACK WindowProcedure (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)                  /* handle the messages */
{
    case WM_CREATE:{
         CreateWindow(TEXT("Button"), TEXT("Generate"),
             WS_CHILD | WS_VISIBLE,
             10, 160, 80, 20,
             hwnd, (HMENU) ID_BUTTON, NULL, NULL
             );
         hwndA = CreateWindow(TEXT("EDIT"), TEXT(""),
             WS_VISIBLE | WS_CHILD | WS_BORDER,
             190, 130, 70, 15,
             hwnd, (HMENU) ID_TEXTBOX, NULL, NULL
             );    
         hwndB = CreateWindow(TEXT("EDIT"), TEXT(""),
             WS_VISIBLE | WS_CHILD | WS_BORDER,
             260, 130, 70, 15,
             hwnd, (HMENU) ID_TEXTBOX, NULL, NULL
             );
         break;
         }
    case WM_COMMAND:{

         if (LOWORD(wParam) == ID_BUTTON) {
            char wot[256];

            GetWindowText(hwndA, wot, 4);
            SetWindowText(hwndB, wot);
            }
         }

         break;
         }
    case WM_DESTROY:
        PostQuitMessage (0);       /* send a WM_QUIT to the message queue */
        break;
    default:                      /* for messages that we don't deal with */
        return DefWindowProc (hwnd, message, wParam, lParam);
}

return 0;
}

The AT.c is just big, yet very simple calculating program. Like

#include<stdio.h>
#include<stdlib.h>
#include<math.h> 

int a, b, c;

main(){
   scanf("%d", &a);
   scanf("%d", &b);
   printf("\n%d", a+b);
   system("pause");
}

just with bunch of ifs and loops.