Using distinct with sum()

Hello,

I've trying to sum up distinct columns. But it's summing them prior to
the distinct taking effect.

--
CREATE TABLE t1 (
id integer,
value1 integer,
value2 integer,
value3 integer
);

INSERT INTO t1 (id,value1,value2,value3) VALUES (1,100,300,null);
INSERT INTO t1 (id,value1,value2,value3) VALUES (1,100,300,1);
INSERT INTO t1 (id,value1,value2,value3) VALUES (1,100,300,2);
INSERT INTO t1 (id,value1,value2,value3) VALUES (2,200,400,3);
INSERT INTO t1 (id,value1,value2,value3) VALUES (3,400,500,4);

SELECT DISTINCT id,value2 - value1 AS total FROM t1;

id | total
----+-------
1 | 200
2 | 200
3 | 100
--

Now what I'd like to do count(id) and sum(total)

--
SELECT DISTINCT count(id),sum(value2 - value1) AS total FROM t1;

count | total
-------+-------
5 | 900
--

I can get count() working how like by doing count(DISTINCT id) but how
do I do get a query to produce the following output (count/sum
post-distinct):

count | total
-------+-------
3 | 500

Thank you in advance,
- A Gilmore