Step 1 – Select Value Rules and then select the rule with "sum of".
Step 2 - Determine how many values you will need included in your sum. Then click on the wheel icon. Drag over the item piece from the left into the list on the right. Repeat this until you have the correct number of items you will need for your sum. If you need to remove an item, simply click back on the wheel icon and drag an item piece from the right back to the left until you get the ‘x’ and release. This will reduce the number of items from the list by one.
Step 3 - Finally, click on the drop down for each item to choose the fields to include.