A Simple Calculator Program:

def add(a, b):
return a + b

def subtract(a, b):
return a - b

def multiply(a, b):
return a * b

def divide(a, b):
if b != 0:
return a / b
else:
return "Error: Division by zero"

def calculator():
print("Select operation:")
print("1. Add")
print("2. Subtract")
print("3. Multiply")
print("4. Divide")

choice = input("Enter choice(1/2/3/4): ")

if choice in ['1', '2', '3', '4']:
    num1 = float(input("Enter first number: "))
    num2 = float(input("Enter second number: "))
    
    if choice == '1':
        print(f"Result: {add(num1, num2)}")
    elif choice == '2':
        print(f"Result: {subtract(num1, num2)}")
    elif choice == '3':
        print(f"Result: {multiply(num1, num2)}")
    elif choice == '4':
        print(f"Result: {divide(num1, num2)}")
else:
    print("Invalid Input")

if name == "main":
calculator()