conversion between numerical systems

# Conversion between numerical systems.

keep_running = False
while (keep_running == False):
    print()
    print("You can select one word of the menu, writing the word:")
    print("binary-denary     denary-binary     bye")
    print()
    command = input("How can I help you? ")
    #print()

    if command != "binary-denary" and command != "denary-binary"  and command != "bye":
        print("Sorry, I don't understand.")

    elif command == "binary-denary":
        binary = input("Input a number in binary:")
        denary = 0
        for digit in binary:
            #A left shift in binary means x2
            denary = denary*2 + int(digit)
        print("Your denary number is: " + str(denary))

    elif command == "denary-binary":
        denary = int(input("Input a denary number:"))
        binary=""
        while denary > 0:
            #A left shift in binary means /2
            binary = str(denary%2) + binary
            denary = denary//2
        print("Your binary number is: " + binary)

    elif command == "bye":
        keep_running = True