diff --git a/nemu/src/monitor/sdb/addrexp.y b/nemu/src/monitor/sdb/addrexp.y
index 82407e2..2b268ac 100644
--- a/nemu/src/monitor/sdb/addrexp.y
+++ b/nemu/src/monitor/sdb/addrexp.y
@@ -26,7 +26,7 @@ expression
     | expression '-' expression { $$ = $1 - $3; }
     | expression '*' expression { $$ = $1 * $3; } 
     | expression '/' expression { $$ = $1 / $3; if($3 == 0) perror("Devide by 0\n");}
-    // | '-' number { $$ = -$2; }
+    | '-' number { $$ = -$2; }
     | '(' expression ')' { $$ = $2; }
 
 number