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
4
Output
2
Input
5
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：
1 #include <bits/stdc++.h> 2 using namespace std; 3 int main(){ 4 long long n; 5 cin>>n; 6 if(n&1)cout<<(n/2-n)<<endl; 7 else cout<<(n/2)<<endl; 8 return 0; 9 }