diff --git a/octa/range.h b/octa/range.h
index 5b8cacf..43aa64d 100644
--- a/octa/range.h
+++ b/octa/range.h
@@ -333,12 +333,12 @@ public:
         return octa::move(tmp);
     }
 
-    RangeHalf operator+(RangeDifference<T> n) {
+    RangeHalf operator+(RangeDifference<T> n) const {
         RangeHalf tmp(*this);
         tmp.add_n(n);
         return octa::move(tmp);
     }
-    RangeHalf operator-(RangeDifference<T> n) {
+    RangeHalf operator-(RangeDifference<T> n) const {
         RangeHalf tmp(*this);
         tmp.sub_n(n);
         return octa::move(tmp);