Activities Stack Issue
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I have two sets of Activities suppose 3 Activities each set, (A1,B1,C1 || A2,B2,C2) I start my App from A1 then -> B1 -> C1 here I want to jump from C1 to -> A2 and at A2 if I press back it should exist the App and not put me back for C1, then from A2 I navigate to -> B2 -> C2. 

So it is basically I want to change the starting Activity, it is like I have two Apps in one App and when I flip to the second App I have to clear the Activity Stack. Is that possible? Any Ideas?