selecting a pending order when it reaches the price

[Farvardin Faili]

hi i have a simple question... i want to select a pending order when it reaches its price for example in the code below 2 orders gets opened and the second one is a buystop as you see....i want to modify the stoploss of the first order that is a buy after the second order reaches its price(Ask+20*Point)....

if(open_pair(Symbol())<2)//open orders based on the current symbol
{
   int ticket=OrderSend(Symbol(),OP_BUY,0.02,Ask,5,0,Ask+30*Point);
   int ticket2=OrderSend(Symbol(),OP_BUYSTOP,0.02,Ask+20*Point,5,0,Ask+30*Point);
   if(ticket2>0)
     bool modify=OrderModify(ticket,OrderOpenPrice(),Ask-30*Point,OrderTakeProfit(),0);
   }
  }

the problem is it just modifies the order when buystop simply opens not when it reaches its price can you help me how to solve that....thank you in advance