Risolvere un problema nelle stringe Python.

Ho iniziato a programmare con Python da ieri mattina, mi sono visto due ore (la metà) di un tutorial per principianti e ho provato a fare questo codice che sfortunatamente non mi fa allegare.
Non funziona una parte e non so perchè, qualcuno potrebbe darmi una mano? (In inglese ho descritto un po' il problema)
Grazie d'anticipo.
Masso27

CODICE

#works normally
answer = input("Welcome to the calculator! Do you want to add or multiply? ")

#this is the problem, it doesn't respond to the 13th string, as if it didn't exist. Both add and mul show even if I say
#that only one of them should show, it depends on the answer of the user (see string 15-16-17-18-19-20)
add = num1 = input("First number you want to add: ")
add = num2 = input("Second number you want to add: ")
result = float(num1) + float(num2)

mul = num3 = input("First number you want to multiply: ")
mul = num4 = input("Second number you want to multiply: ")
result2 = float(num1) * float(num2)

#doesn't work and I don't know why.
if answer == "add":
add
if answer == "multiply":
mul
else:
print("You didn't write anything!")

#works normally
def congratulations():
print("Thank you for using my calculator!")

def the_result():
print("The result is the following: ")

def final_numbers():
if answer == "add":
print(result)
result_bigger()
else:
print(result2)
result2_bigger()

def result_bigger():
if result > 0:
print("your number is bigger than 0!")
else:
print("your number is smaller than 0!")

def result2_bigger():
if result2 > 0:
print("your number is bigger than 0!")
else:
print("your number is smaller than 0!")

#works normally
the_result()
final_numbers()
congratulations()