diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..b3f9bb3
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1 @@
+attrs==23.1.0
diff --git a/src/bookstore/isbn.py b/src/bookstore/isbn.py
new file mode 100644
index 0000000..0c8e1ce
--- /dev/null
+++ b/src/bookstore/isbn.py
@@ -0,0 +1,12 @@
+from attrs import define, field, validators
+
+
+# TODO: create checksum method
+@define(frozen=True)
+class ISBN(str):
+    number: str = field(converter=str, validator=validators.matches_re(r"^\d{10}$|^\d{13}$"))
+
+    def __str__(self):
+        if len(self.number) == 10:
+            return f"{self.number[:9]}-{self.number[9:]}"
+        return f"{self.number[:3]}-{self.number[3:4]}-{self.number[4:6]}-{self.number[6:12]}-{self.number[12:]}"