牛牛学数列

#include <iostream>

using namespace std;

int main()
{
    int n;
    cin >> n;
    int flag = 1; 
    int sum = 0;
    for(int i = 1; i <= n; i++) {
        sum += i * flag;
        flag = -1 *flag; // 调节值得正负
    }
    cout << sum;
    return 0;
}