There are given segments $AB, CD$ and a point O such that no three of the points $A, B, C, D, O$ are on one straight line. The endpoint of a segment is marked if the straight line passing through it and O does not intersect another segment. How many marked endpoints are there? Please provide the sum of all possible numbers of marked endpoints.

Let's think step by step and output the final answer within \boxed{}.
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