digital logic subtrator

Hello...
I am a beginner to digital electronics. I have designed and studied the adder circuits. I tried my best to study how to use these adder circuits to design subtractor circuits (using complementary addition). Probably the subtractor ciruits use the logical "if" circuit. I need some help relating this topic. Kindly somebody give me an idea how to design a simple subtractor circuit. Thanks. (From: Atif Shabir).

just encode the number as 2s complement and the answer is in 2s complement - nothing extra to be done