PL SQL block quoted string not properly terminated

I have written a simple PL SQL block , i am getting this error Please tell me whats the problem in this block ??

declare
a 开发者_StackOverflow社区number:=&a;
b number:=&b;
c number:=&c'
begin
if(a>b) then
DBMS_OUTPUT.PUT_LINE(a);
else
 DBMS_OUTPUT.PUT_LINE(b);
end if;
end;
/

---

Try (as Codo also said):

DECLARE 
   a NUMBER := &a; 
   b NUMBER := &b; 
   c NUMBER := &c; 
BEGIN 
   IF (a > b) 
   THEN
      DBMS_OUTPUT.PUT_LINE(a); 
   ELSE  
      DBMS_OUTPUT.PUT_LINE(b); 
   END IF; 
END; 
/