Cannot convert aggregated result to number after write a super metric

I wrote follow super metric actually I need this output but

{Cluster Compute Resource: Memory|Total Capacity (KB), depth=1}-({Cluster Compute Resource: Memory|Consumed (KB), depth=1}+{Cluster Compute Resource: Memory|Reserved Capacity (KB), depth=1})

but show follow error :

Cannot convert aggregated result to number.

How can get output from this super metric with GB instead of KB ?