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

4

Output

2

Input

5

Output

-3

Note

f(4) =  - 1 + 2 - 3 + 4 = 2

f(5) =  - 1 + 2 - 3 + 4 - 5 =  - 3

问题分析；注意n很大要用long long int，利用等差数列求和公式

AC代码：

#include<iostream>
using namespace std;
int main()
{long long int n,a,b,c;while (cin >> n){if (n % 2 == 0){c = n / 2;a = c * 2 + c * (c - 1);b = c + c * (c - 1);cout << a - b << endl;}else{c = (n - 1) / 2;a = c * 2 + c * (c - 1);b = c + c * (c - 1)+n;cout << a - b << endl;}}
}