#include <iostream>
using namespace std;

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