How to get Total Sum of all entries in a Core Data attribute?

I've got a Core Data Data Model with an Entity and several Attributes.

One of these Attributes is an Integer 16 (NSDecimalNumber) called

numberOfSeconds

I want to find the sum of all the entries I have in my data model from this Attribute.

In other words: I'll have many user entries in this Attribute. Lets say the user creates 3 entries with these specifics:

1) numberOfSeconds = 4

2) numberOfSeconds = 2

3) numberOfSeconds = 5

I'm hoping to be able to grab the total sum which should equal 11.

Appreciate any help in this!