def , add number or subtract

def do_calculation():
    print("lets " + command + " some numbers")
    input1 = input("Number 1> ")
    input2 = input("Number 2> ")
    number1 = int(input1)
    number2 = int(input2)
    if command == "add"  :
        result = number1 + number2
        k = "+"
    elif command == "subtract" :
        result = number1 - number2
        k ="-"
    output = str(result)
    print(input1 + k + input2 + " = " + output)

finished = False
while finished == False :
    print("Hi, I am Marvin, your personal bot.")
    command = input("How can I help? ")
    if command == "add":
        do_calculation()

    elif command == "subtract":
        do_calculation()

    elif command == "bye" :
        finished = True
    else :
        print ("Sorry I do not understand this " )