diff --git a/Lesson_3/task1.py b/Lesson_3/task1.py
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+# Lesson 3 Task 1: calculator with functions
+# Author: Stanislav Mykhailenko
+# License: Unlicense
+
+# Return codes:
+# 0 - OK
+# 1 - Invalid numbers entered
+# 2 - Division by zero
+# 3 - Attempting to use a root with non-natural degree
+# 4 - Attempting to extract a root of a negative number with an even degree
+# 5 - Invalid operation entered
+
+import sys
+
+def add (a, b): return a + b
+def substract(a, b): return a - b
+def multiply(a, b): return a * b
+def divide(a, b):
+	if b == 0:
+		print("Division by zero.")
+		sys.exit(2)
+	else:
+		return a / b
+def square(number): return number**2
+
+def isNatural(number): return False if number < 1 or not number.is_integer() else True
+def isEven(number): return True if (number % 2) == 0 else False
+def isNegative(number): return True if number < 0 else False
+
+def root(number, degree):
+	if not isNatural(degree):
+		print("Can't extract a root with non-natural degree.")
+		sys.exit(3)
+	elif isEven(degree) and isNegative(number):
+		print("Can't extract a root of a negative number with an even degree.")
+		sys.exit(4)
+	else:
+		if number < 0:
+			return -abs(number ** (1 / degree))
+		else:
+			return number ** (1 / degree)
+
+try:
+	numberA = float(input("Enter number A: "))
+	numberB = float(input("Enter number B: "))
+except ValueError:
+	print("Invalid numbers entered.")
+	sys.exit(1)
+
+print(
+'''
+Valid operations:
+Addition: +
+Subtraction: - or −
+Multiplication: * or ×
+Division: / or ÷
+Root: root or √ (extracted from the first number, with the second number being its degree)
+Square: square or ² (of both numbers)''', end="\n\n"
+)
+
+operation = input("Enter operation: ")
+
+if operation == "+":
+	print(add(numberA, numberB))
+elif operation == "-" or operation == "−": # ASCII hyphen-minus or Unicode minus
+	print(substract(numberA, numberB))
+elif operation == "*" or operation == "×": # ASCII asterisk or Unicode multiplication sign
+	print(multiply(numberA, numberB))
+elif operation == "/" or operation == "÷": # ASCII slash or Unicode division sign
+	print(divide(numberA, numberB))
+elif operation == "root" or operation == "√": # Unicode radical symbol
+	print(root(numberA, numberB))
+elif operation == "square" or operation == "²": # Unicode superscript 2
+	print("Number A square: " + str(square(numberA)) + "\nNumber B square: " + str(square(numberB)))
+else:
+	print("Invalid operation.")
+	sys.exit(5)