Sql query to convert nvarchar to int

I have to query for total amount of a column using an aggregate function. The column data type is NVARCHAR(MAX). How can I convert it to Integer?

I have tried this:

  SELECT SUM(CAST(amount AS INT)),
         branch 
    FROM tblproducts  
   WHERE id = 4
GROUP BY branch

...but I'm getting:

Conversion failed when converting the nvarchar value '3600.00' to data type int.