HDU Sum Problem (一道神坑的水题)

Problem Description

Hey, welcome to HDOJ(Hangzhou Dianzi University Online Judge).

In this problem, your task is to calculate SUM(n) = 1 + 2 + 3 + ... + n.

 

 

Input

The input will consist of a series of integers n, one integer per line.

 

 

Output

For each case, output SUM(n) in one line, followed by a blank line. You may assume the result will be in the range of 32-bit signed integer.

 

 

Sample Input

1

100

 

Sample Output

1

 

 

5050

 

巨坑无比 

我直接用等差公式 WA到根本停不下来

 

题目中标红的字是关键 

n*(n+1)可能会超出32位

 

所以先 n/2(n%2==0)

或者(n+1)/2  ---(n%2==1)

 

代码自己写吧