Sum string field

Hi, i`m using a spredsheet model to make a report. The model has this fields:

- DeviceName

- Country

- City

- Store (string)

- TImeConnected (int)

I made an object with this hierarchy (for drill): Country - City - Store - DeviceName

Also, i made two card objects, one to showing the sum of TimeConnected field, and other, showing the Sum of Sore field. Here is the problem, I wanna show the store field sum as a number, and when i drill into the object, modify this value, as i could make it work with the TimeConnected field (this was easier because it is an int field, so i show the sum)

How can i make it work?
Thanks in advance!