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

/**************************************************************
	Problem: 1110
	User: linzihang
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/