First I am NOT a developer and I always write my groovy expression with what I find here in Atlassian answers. But not this time 
I am trying to set a number custom field with a negative value. It is only done if another custom field has the "Non" value. Plus I will have to make sure that the custom field number is not already negative.
I tried different combinaison and I am not able to change the value of the field named Montant.
issue.getCustomFieldValue("customField_19313")
or
issue.get("customField_19313") * -1 I really don't know what to try next. Any help will be appreciate.
image2017-2-13 15:3:14.png