diff --git a/arithmetic_arranger.py b/arithmetic_arranger.py
index 6f59604..a6abfba 100644
--- a/arithmetic_arranger.py
+++ b/arithmetic_arranger.py
@@ -6,7 +6,7 @@ def arithmetic_arranger(problems, val=False):
 
     # list of all operations in str format
     operations = list(map(lambda x: x.split()[1], problems))
-    if set(operations) != {'+', '-'} and len(set(operations)) != 2:
+    if set(operations) not in [{'+'}, {'-'}, {'+', '-'}] and len(set(operations)) > 2:
         arranged_problems = "Error: Operator must be '+' or '-'."
         return arranged_problems