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Why is my steam game(HL2 dm) not launching?

well i juss downloaded Half-Life 2 Death match. i dont have normal half life 2. but ive heard that its a stand alone game. when i click it it saying preparing to launch and then the cancel button goes dark and it goes away. no launch no nothing, i have the requirments and i also have ran portal perfectly. i dont know whats wrong. any thoughts