To send sub ID from prev response -1

Hi

I have one scenario.

In purchase, I get subscription-id in response. e.g. If my subscripton-id is 78, there is one more sub with id 77 in DB

So I have to pass sub-id as 77 for further actions. can you please let me know how to achieve this in Ready API 2.0

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<request>
  <freeze-sub>
      <msisdn>447717887733</msisdn>
      <subscription-id>${Preconsition - Puchase pAlt#ResponseAsXml}${Preconsition - Puchase #ResponseAsXml#//response[1]/subscription-id[1]}</subscription-id>
      <action>FREEZE</action>
    </freeze-sub>
</request>

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Note: If usig thisfollowing, i gets sub-id as 782, then i have to send in request as 781

${Preconsition - Puchase pAlt#ResponseAsXml}${Preconsition - Puchase #ResponseAsXml#//response[1]/subscription-id[1]}

Can you please help me in this

Regards

Vishal