In the following pl sql program what is the error ? Is it a compilation error?

Declare
 a VARCHAR2(10);
 b NUMBER;
BEGIN 
 a:=&empname;
 b:=&empno;
 DBMS_OUTPUT.PUT_LINE('WELCOME'a||','||b||);
END;

When I am running it on oracle sql developer, it is showing some some which I can not able to figure out.