#include <bits/stdc++.h>
using namespace std;
int main(){
	int i,sum=0;
	cin>>i;
	for(int j=1;j<=i;j+=2)sum+=j;
	cout<<sum;
	return 0;
}

/**************************************************************
	Problem: 1110
	User: zhouhongyi
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/