making "one-way" platform

Hello.I'm expanding the unity's "2D Gameplay Tutorial " to inprove my coding skill. I want to make the platform like this...

1. It won't work as a platform if the player(character controller)'s y position are increasing.

2. It will work as a platform if the player's y position are decreasing.

This is my code attached in the platform.But it just works as a normal platform . Please improve this code.Thanks in advance.

 function OnCollisionStay(col:Collision){
    if(col.relativeVelocity.y>0)
      enabled=false;

}