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
算法要求求-1+2-3+4······，所以可以分为两类情况，奇数和偶数，两两为一组然后计算。
···

include<iostream>

using namespace std;
int main()
{
long long n;
cin >> n;
if (n % 2 == 0)
cout << n / 2;
else
cout << -n / 2 - 1;
return 0;
}
···