I have a CSF-1099R form. Box 2a (Taxable amount) is "Unknown." Would the taxable amount be Box 1 or would it exclude Box 5, which is amount deducted for health insurance.

This is for a civil service pension. I don't know how to answer the question "Describe the taxable amount." How do I know if 1099R says "unknown"? Box 5 is amount deducted for health insurance. Is that taxable?