simple

                           Simple maths
Let + represent (Friend), and - represent (Enemy). In Mathematics;
+ multiplied by + = + 
+ multiplied by  - = - 
-  multiplied by + = - 
-  multiplied by- = + 

Therefore, replacing duely,

Friend(+) of your Friend(+) = YOUR FRIEND.
Friend(+) of your Enemy(-) = YOUR ENEMY.
Enemy(-) of your Friend(+) =YOUR ENEMY.
Enemy(-) of your Enemy(-) = YOUR FRIEND.