#include <iostream>
using namespace std;

int main() {
    
    int n;
    cin >> n;
    int sum = 0;
    
    // write your code here......
    
    if (n<=1000 && n>=1)
        {
            for (int i=1; i<= n; i++)
                {
                    if (i%2 == 0)
                        sum += i;
                }
        }
    cout << sum << endl;
    
    return 0;
}