B - Calculating Function

Problem description

For a positive integer n let's define a function f:

f(n) = - 1 + 2 - 3 + .. + ( - 1)ⁿn

Your task is to calculate f(n) for a given integer n.

Input

The single line contains the positive integer n (1 ≤ n ≤ 10¹⁵).

Output

Print f(n) in a single line.

Examples

Input

Output

Input

Output

-3

Note

f(4) = - 1 + 2 - 3 + 4 = 2

f(5) = - 1 + 2 - 3 + 4 - 5 = - 3

解题思路：简单求前n项和，分为奇数和偶数两种情况，水过！

AC代码：

 #include <bits/stdc++.h>

 using namespace std;

 int main(){

     long long n;

     cin>>n;

     if(n&)cout<<(n/-n)<<endl;

     else cout<<(n/)<<endl;

     return ;

 }

巴特西

B - Calculating Function

Problem description

Input

Output

Input

Output

Input

Output

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