Get the sum of child's tables fields in postgresql

Question

I have table scheme as below:

Table A:

id  field_a1:integer   
1  100

Table B

 id  a1_id:integer  field_b1   
 1   1              201        
 2   1              202        

Table C

id  a1_id:integer  field_c1   
1   1              200

I need sql query to get the SUM of Table A and B fields so result should be as bellow

SQL results(should be):

table_A_field_a1   total_sum(field_c1+field_b1)
100                603(200+201+202)

my sql query is

SELECT (SUM(field_c1)+SUM(field_b1)) total_sum, a1.* 
FROM A as a 
LEFT JOIN B as b on b.a1_id = a.id
LEFT JOIN C as c on c.a1_id = a.id
GROUP BY a.id

But i am getting total_sum = 803 but it should be 603