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

/**************************************************************
	Problem: 1003
	User: hongguangxi
	Language: C++
	Result: Accepted
	Time:6 ms
	Memory:2072 kb
****************************************************************/