#include<bits/stdc++.h>
using namespace std;
int main(){
	int n;
    cin >> n;
    int total = 0;
    for (int i = 1; i <= n; i += 2) {
        total += i;
    }
    cout << total << endl;
    return 0;
}
/**************************************************************
	Problem: 1110
	User: panyuchen
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/