How do you use variables in a simple PostgreSQL script?

DO $$
DECLARE  
   a integer := 10;  
   b integer := 20;  
   c integer;  
BEGIN  
   c := a + b;
    RAISE NOTICE'Value of c: %', c;
END $$;

https://stackoverflow.com/questions/766657/how-do-you-use-variables-in-a-simple-postgresql-script