What do negative values ​​in Appstats mean?

Appstats shows negative values ​​for some of my queries on my Google App Engine development server. For instance,

real=3604ms api=0ms overhead=1ms (13 RPCs, cost=-3588220, billed_ops=[DATASTORE_READ:4, DATASTORE_WRITE:-35885])
                        #RPCs     Cost
memcache.Get                4        0  
memcache.Set                3        0  
datastore_v3.RunQuery       2      210       DATASTORE_READ:3
datastore_v3.Get            1       70       DATASTORE_READ:1
datastore_v3.Put            1 -3588500       DATASTORE_WRITE:-35885
memcache.Delete             1        0  
search.DeleteDocument       1        0  

Is negative value the same as positive, but for delete operations?