Isn't it simple counting?

Let $A_{1}A_{2}A_{3}A_{4}.....A_{21}$ be a 21-sided regular polygon inscribed in a circle with centre $O$ . How many triangles $A_{i}A_{j}A_{k}$ , $0<i<j<k<22$ , contain the point $O$ in their interior.

NOTE: This is not an original problem