def start():
      var sPrice, minPrice, numShares

      while 0<=minPrice and minPrice<sPrice:
         prob(1,3):
            sPrice=sPrice+1
         else:
            sPrice=sPrice-1
         numShares = unif(0,20)
         while numShares>0:
             numShares=numShares-1
             tick(sPrice)