#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,sum=0;
	cin>>n;
	for(int i=1;i<=n;i+=2){
		    sum+=i;
	}
	cout<<sum;
}
/**************************************************************
	Problem: 1003
	User: linmiaoling
	Language: C++
	Result: Accepted
	Time:6 ms
	Memory:2072 kb
****************************************************************/