Need help with adding variables

I need to add two variables together
here is the current code

private $gamestarttime = 5;
private $gamewaittime = 5;
private $minute = $this->gamewaittime + $this->gamestarttime;

The third time.
You can't resolve variables when declaring fields.

what does this mean??????????

Maybe use:

PHP:

public function onEnable(){
   $this->minute = $this->gamewaittime + $this->gamestarttime;
}