#include<bits/stdc++.h>
using namespace std;
int n,s;

int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
	if(i%2==1 ) s=s+i;
	


	
	
	}
 
cout<<s;

	return 0;
}

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