牛牛学数列

#include<iostream>
using namespace std;


int main()
{
    int n;
    int sum = 0;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        if (i % 2 == 0)
            sum -= i;
        else
            sum += i;
    }
    cout << sum << endl;
    return 0;
}