how to decrease a fixed number of coins after every click?

I am working with a small project where the users can get coinsby watching adds for unlocking some walpapers....I want to set the coin limit 500 for unlocking a walpaper......if a user has less then 500 coin, he/she can't unlocking the walpaper... Also I want...after unlocking the walpaper his/her total coin limit will decrease the same amount of the requirement....I dont know how to write the conditions in the if statement.... Plez help guys.....

       If(score>=500)
          {   
        }else
            {    
          }