Your question is unclear. Did you only pay for insurance for 5 months and then the insurance was cancelled? Or did you buy insurance through the Health Insurance Marketplace? If the former, just list that amount as the amount you spent for health insurance.
If the latter, make sure to include in the return the "Form 1095-A, Health Insurance Marketplace Statement" you should have received in the mail from your health insurer. If you didn't receive it or can't find it, you should be able to download a copy from your health insurance online account.