A - Sum(1.5.1)

Description

Your task is to find the sum of all integer numberslying between 1 and N inclusive.

Input

The input consists of a single integer N that isnot greater than 10000 by it's absolute value.

Output

Write a single integer number that is the sum of allinteger numbers lying between 1 and N inclusive.

Sample Input

-3

output

-5

代码：

#include <iostream>
 using namespace std;
 int main()
 {
     int n;
     cin>>n;
     if(n>0)
         cout<<(n*(n+1))/2;
     else
     cout<<((2-n)*(n+1))/2<<endl;
     return 0;
 }