#include <iostream>
using namespace std;

int main() {
    int n, sum = 0;
    cin >> n;

    for (int i = 1; i <= n; i += 2) {
        sum += i;
    }

    cout << sum << endl;
    return 0;
}
/**************************************************************
	Problem: 1003
	User: fuyijun
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/