Increment/Decrement inventory_quantity

[Sako]

We have a Cashe desk connected to Shopify, which sells in parallel the same products of Shopify.

How is it possible to inform Shopify about the changes in quantity?

According to the Documentation of Product APIs </a> , I see it is only possible to set the inventory_quantity to a new value.

My question is, how I can tell Shopify to increment the inventory_quantity for example with +2?

[Sako]

Of course I can  GET  the inventory_quantity of the product apply +2 to it an then POST the new inventory_quantity, but I want to avoid this (if there is any alternative).