sum coustm field in all child story under an epic and save the result in another field in the epic

[IDO GAFSON]

i create an automation that calculate "burn story point" from the "story point" field by different status , in the end of the automation i need to run script that sum all the "burn story point" from all the story under the epic and save the result under custom field in the epic 
i can't use lookup function because i use JIRA DC