Puzzleup 2016 (12) Last Digit - 拼圖

題目網址: http://www.puzzleup.com/2016/
http://www.puzzleup.com/2016/puzzle/?12
答題時限: 10月13日７ＰＭ－比賽結束（約12月14日）
加分時限: 10月13日７ＰＭ－10月19日６：５９ＰＭ

答對可得基本分１００分。答案可上傳５次，每改１次答案從基本分扣２０分。　
比賽期間內可隨時上傳答案，加分時限內答對第ｎ天加（６－ｎ）分　 　　 　　
另依題目的難易有額外加分（如有８０％的人這題答錯，答對者加８０分）　　

◆LAST DIGIT

What is the last digit of the result of this addition operation?

以下和式的末位數字為何？

1 2 3 (10^100 - 1) (10^100)
1 + 2 + 3 + ... + (10^100 - 1) + (10^100)

====

¯\_(ツ)_/¯

--
'You've sort of made up for it tonight,' said Harry. 'Getting the
sword. Finishing the Horcrux. Saving my life.'
'That makes me sound a lot cooler then I was,' Ron mumbled.
'Stuff like that always sounds cooler then it really was,' said
Harry. 'I've been trying to tell you that for years.'
-- Harry Potter and the Deathly Hollows, P.308

--