Need to sum two fields in document

[YasserAndWesam]

i want to sum two fields in document using this code but raise an error
org.apache.solr.search.SyntaxError: Expected ')' at position 32 in 'sum(field(10638817_EXACT_PARTS),field(10638812_EXACT_PARTS))'
.

http://10.68.20.139:5080/solr/reports_core/select?q=10638817_EXACT_PARTS:* OR 10638812_EXACT_PARTS:* &rows=0&json.facet={STAR_RATING:{terms:{field:STAR_RATING,facet:{Totalsum:'sum(field(10638817_EXACT_PARTS),field(10638812_EXACT_PARTS))'}}}}

can you help me to sum two fields .