diff --git a/SL_2/s2_a4_a.py b/SL_2/s2_a4_a.py
new file mode 100644
index 0000000..7ead30f
--- /dev/null
+++ b/SL_2/s2_a4_a.py
@@ -0,0 +1,16 @@
+import re
+
+
+def normalize_number(phone_number):
+    pattern = re.compile("([\+]*[1][ -])*(\(*[2-9][0-9][0-9]\)*)[- .]([0-9][0-9][0-9])[ -.]([2-9][0-8][0-9][0-9])")
+
+    if not pattern.match(phone_number):
+        raise ValueError("Ungültige Telefonnummer")
+
+    normalized_number = re.sub("\+*\(*\)*", "", phone_number)
+    normalized_number = re.sub("[ .]", "-", normalized_number)
+
+    if normalized_number[0] != str(1):
+        normalized_number = "1-" + normalized_number
+
+    return normalized_number
diff --git a/SL_2/s2_a4_b.py b/SL_2/s2_a4_b.py
new file mode 100644
index 0000000..2f3a082
--- /dev/null
+++ b/SL_2/s2_a4_b.py
@@ -0,0 +1,33 @@
+import unittest
+from s2_a4_a import normalize_number
+
+
+class TestPhoneNumber(unittest.TestCase):
+    test_correct_numbers = [
+        "+1 223-456-7890",
+        "1-223-456-7890",
+        "+1 223 456-7890",
+        "(223) 456-7890",
+        "1 223 456 7890",
+        "223.456.7890",
+        "1-989-111-2222"
+    ]
+
+    test_wrong_numbers = [
+        "+1 123-234-3333",
+        "1 222-333-1111",
+        "222-333-9999",
+        "22-333-4444"
+    ]
+
+    def test_given_numbers_correct(self):
+        for numbers in self.test_correct_numbers[:-1]:
+            self.assertEqual("1-223-456-7890", normalize_number(numbers))
+        self.assertEqual("1-989-111-2222", normalize_number("1-989-111-2222"))
+
+    def test_number_exception(self):
+        for numbers in self.test_wrong_numbers:
+            with self.assertRaises(ValueError) as context:
+                normalize_number(numbers)
+
+                self.assertTrue('This is broken' in context.exception)