# 0J/QsNGA0YXQvtC80LXQvdC60L4=
def f(n):
if n <= 0:
return 0
elif n == 1:
return 1
else:
a, b = 0, 1
for _ in range (2, n):
c=a+b
a=b
b=c
return b
print(f(20))
IyAwSi9Rc05HQTBZWFF2dEM4MExYUXZkQzYwTDQ9CmRlZiBmKG4pOgoJaWYgbiA8PSAwOgoJCXJldHVybiAwCgllbGlmIG4gPT0gMToKCQlyZXR1cm4gMQoJZWxzZToKCQlhLCBiID0gMCwgMQoJCWZvciBfIGluIHJhbmdlICgyLCBuKToKCQkJYz1hK2IKCQkJYT1iCgkJCWI9YwoJCXJldHVybiBiCnByaW50KGYoMjApKQoJCQkKCQk=