O-Level Maths and Add Maths---Difference???

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Well i want to give Add Math paper for O-level but first i want to know whats the difference between O-level Maths and Add Maths??
My Math is very good overall so i wanted to know if i can give the paper or not and one more thing......suppose i have given my O-level Math paper and have clear concepts about everything,about how much time would it take me to completely study Add Maths if i have completed the "normal" Maths?