Autohotkey: how to declare global variable after #ifwinactive

When I declare a global variable after #ifwinactive, an error message pop up showing that the variable is not declared.

I have a long script where there are many #ifwinactive ahk_exe xxx. Below those codes, I want to declare a global variable. But that doesn't work. Whenever I run the script, it warns me the global variable is not declared. Here's a brief of my script.

On the top of the script I have these environment settings:

#NoEnv  
#Warn  
SendMode Input 
SetWorkingDir %A_ScriptDir%
SetTitleMatchMode, 2

Then I have many window-specific hotkeys:

#ifwinactive ahk_exe Explorer.EXE
;some hotkeys
#ifwinactive ahk_exe WINWORD.EXE
;some extra hotkeys
;etc.

At the bottom, I try to declare a global variable and this is where the error occurs:

#ifwinactive
a := 1
^p::
    if (a != 1){
        a := 1
    } else {
        a := 0
    }
return 

#if winactive("- YouTube -") and a != 1
    p::msgbox,Yes
#if a = 1 and winactive("- YouTube -")
    p::msgbox,no
#if