Android is a mobile operating system developed by Google. Android is designed primarily for touchscreen mobile devices such as smartphones and tablet computers. The OS uses touch inputs that loosely correspond to real-world actions, like swiping, tapping, pinching, and reverse pinching to manipulate on-screen objects, and a virtual keyboard. As of 2011, Android has the largest installed base of any mobile OS and as of 2013, its devices also sell more than Windows, iOS, and Mac OS devices combined. As of July 2013 the Google Play store has had over 1 million Android apps published, and over 50 billion apps downloaded. A developer survey conducted found that 71% of mobile developers develop for Android. Question: What is the total number of Android phones sold?