都是1000以内偶数的和，为什么结果不一样

>>> while True:

if num>1000:

break

if num%2==0:

sum=sum+num

num=num+1

>>> print(sum)

250500

>>> num=0

>>> sum=0

>>> while num<=1000:

num=num+1

if num %2==1:

continue

sum=sun+num

Traceback (most recent call last):

  File "<pyshell#17>", line 5, in <module>

    sum=sun+num

NameError: name 'sun' is not defined

>>> while num<=1000:

num=num+1

if num %2==1:

continue

sum=sum+num

>>> print(sum)

250498

都是1000以内偶数的和，为什么结果不一样