How to add custom attribute which accepts decimal numbers?

I want to add a new field to the task and defect work items in Scrum process template. The field is used to collect information about the hours how much customers will be invoiced. I am able to add custom attributes from the process configuration feature, but i am not able to find correct data type for this kind of usage. Basically the field should accept decimal numbers (long, double for example), which can be further used for example to calculate the total billable hours.

Could you help me out with this issue, please?