How to sum cells in a column per id in sql server

I am new to Sql server. the sum(datediff(x,y,z)) is giving the sum of both a1 and a2 whereas I want the sum separate for a1 and a2. I dont know how to group it per id. I know that sum will return the sum of both but I want to know how can I separate them.

select email,
    sum(datediff(day, x, y))
from abc
where users.id ='a1'
    or users.id='a2'
group by email, users.id

email|datex|datey|
a1   |1    |2
a1   |1    |3
a1   |2    |5
a2   |2    |7
a2   |3    |7
a2   |1    |8

So what I need

email|diff
a1   |4
a2   |6