#include <iostream>
using namespace std;
int main() {
long int s = 0;
long int n;
cin >> n;
for (int i = 1; i <= n; i++)
if ((i & 1) == 0)
s += i;
cout << s;
return 0;
}
#include <iostream>
using namespace std;
int main() {
long int s = 0;
long int n;
cin >> n;
for (int i = 1; i <= n; i++)
if ((i & 1) == 0)
s += i;
cout << s;
return 0;
}