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依旧”从简单的题目谈起-（HDUACM201403版_02）基础数学题
资源介绍
“依然”从简单题说起：
SUM(n) = 1 + 2 + 3 + ... + n
You may assume the result will be in the range of 32-bit signed integer.
Sample input:
10
Sample output:
55
http://acm.hdu.edu.cn/showproblem.php?pid=1001