Unable to prepare a report for sum quantity of each item

[SSC]

Hello,
i am unable to make it work since last two days.
My Dispatch Table contains fields [Unique ID], [Party Name] , [Site] , [Item], [Quantity].
One Party Name can have multiple sites, and on each site multiple items are dispatched in quantity.

i am trying to create an excel report that shows me the total sum of item quantity dispatched on each site for each party name.

Example:
Party Name Site Item Quantity
Party1 Site1 Item1 10
Party1 Site1 Item2 100
Party1 Site1 Item1 50
Party1 Site2 Item2 100
Party1 Site2 Item1 20
Party1 Site2 Item2 200
Party2 Site1 Item1 10
Party2 Site2 Item1 100

I wish to have the following Result in excel file:
Party Name Site Item Total Quantity
Party1 Site1 Item1 60
Party1 Site1 Item2 100
Party1 Site2 Item1 20
Party1 Site2 Item2 300
Party2 Site1 Item1 10
Party2 Site2 Item1 100

i am not getting what start expression should i use. i used the following start expression but its not working
<<Start: select(Dispatch[Unique ID], true)>>
i am confused what expression will sum up the item quantity as per site and party names. i tried different expressions of unique() too but havent got any result.