I. Introduction
In recent years, Real Time Bidding (RTB) has become the main platform for trading online advertisements (ads). Considering its extremely high turnover, most website owners participate in this business by selling some blocks in their websites to the advertisers. In display advertising, these blocks are called ad slots or ad placements. Basically, selling and buying ad slots are performed through online RTB auctions. These auctions are performed in ad networks or Ad Exchange (AdX) markets in real time and the winner of an auction advertises in corresponding ad slot. The auctions are mainly second price auctions where the winner pays as much as the second highest bid. In second price auctions, the website owner adjusts a reserve price which determines the minimum price of the ad slot. The final price of sold ad slots is the maximum of the reserve price and the second highest bid which is paid to the ad publishers and the ad is shown in the end user's browser [1].