diff --git a/Lesson_2/task4.py b/Lesson_2/task4.py
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+++ b/Lesson_2/task4.py
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+# Lesson 2 Task 4: get number factorial
+# Author: Stanislav Mykhailenko
+# License: Unlicense
+
+# Return codes:
+# 0 - OK
+# 1 - Invalid number entered
+
+import math, sys
+
+def numberError():
+	print("Invalid number entered.")
+	sys.exit(1)
+
+try: number = int(input("Enter a natural number: "))
+except ValueError: numberError()
+
+if number < 1: numberError()
+
+print(str(number) + " factorial is " + str(math.factorial(number)))