Why won't my program substract correctly?

Question

Here it is, hope you can help me:
print("Hi, select the calculation you want to make")
print("Add or Substract")
selection = input(":")
if selection == "Add" or "add":
x = int(input("Enter your first number: "))
y = int(input("Enter the second number: "))
print(x + y)
elif selection == "Substract" or "substract":
a = int(input("Enter your first number: "))
b = int(input("Enter the second number: "))
print(a - b)
else:
print("Unknown Command")

Answer 1

write your input and the result please

Answer 2

If I input Substract, then 5, and then 4 it gives me 9, I think that it executes the "add" lines instead of the "Substract" ones