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

}

/**************************************************************
	Problem: 1110
	User: hongjiaming
	Language: C++
	Result: Accepted
	Time:10 ms
	Memory:2072 kb
****************************************************************/