Ural 1068

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

Input

The input consists of a single integer N that is not greater than 10000 by it's absolute value.

Output

Write a single integer number that is the sum of all integer numbers lying between 1 and Ninclusive.

Sample

input	output
-3	-5

Problem Source: 2000-2001 ACM Northeastern European Regional Programming Contest (test tour)

// Ural Problem 1068. Sum
// Verdict: Accepted  
// Submission Date: 23:26:08 14 Jan 2014
// Run Time: 0.015s
//  
// 版权所有（C）acutus   (mail: acutus@126.com) 
// 博客地址：http://www.cnblogs.com/acutus/
// [解题方法]  
// 简单题，直接计算，注意N为负值

#include<stdio.h>

void solve()
{
    int N;
    scanf("%d", &N);
    if(N ==0) {
        printf("1
");
        return;
    }
    if(N % 2) {
        printf("%d
",(N > 0 ? ((N + 1)/2 * N) : (-1 * (-1 * N + 1)/2 * (-1 * N) + 1))); 
    } else {
        printf("%d
", (N > 0 ? (N/2 * (N + 1)) : (-1 * (-1 * N)/2 * (-1 * N + 1) + 1)));
    }
}

int main()
{
    solve();
    return 0;
}