bet if there is lay amount waiting greater than x

Hi i ave a trigger that back in the favorite, but i want to add the next condition:


1) Only do the back bet, if the amount of money waiting in the lay side its greater that 500.


     Example: i back real madrid at odds 2:00.
                  a) In the lay side there are 5 euros at odds 2.25, and 14e at odds 4. Then dont do that bet.


                  b) In the lay side there are 400 e at odds 2.25 and 145 e at odds 4. Then do that bet.


i need that condition for put. Thanks