The Red Green Sticks

In the tech fest of JEC a game was organized for fun. You are participating in that game .In the game you are given $k$ sticks of red color and $k$ sticks of green color. you have to arrange these sticks linearly to form stack but with a condition. * for every red stick there should be a corresponding green stick before it , it will be called a valid pair of sticks if it satisfies another cond