How to add all non-null values of a metrics in Graphite?

Question

I have a metrics say,

• a.b.c.metric --> 21(value) 12345678(timestamp)
• a.b.c.metric --> 22(value) 12345688(timestamp)
• a.b.c.metric --> 23(value) 12345698(timestamp)

I want to sum all these values and get 66. How can I do that in graphite data store?