diff --git a/pymath/calculus/operator/operator.py b/pymath/calculus/operator/operator.py
index ae0a1ed..6ac72df 100644
--- a/pymath/calculus/operator/operator.py
+++ b/pymath/calculus/operator/operator.py
@@ -180,11 +180,12 @@ class Operator():
             ans = ' '.join([str(i) for i in ans])
         return ans
 
-    def r_parenthesis(self, op, str_join=False):
+    def r_parenthesis(self, opr, str_join=False):
         """ Add parenthesis for rigth operand if necessary """
-        ans = op
+        ans = opr
         try:
-            if op.mainOp.priority < self.priority:
+            if opr.mainOp.priority < self.priority or \
+                (opr.mainOp.name == 'mul' and opr[0] == '-'):
                 ans = flatten_list(["(", ans, ")"])
         except AttributeError:
             # op has not the attribute priority