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

	return 0;
}

/**************************************************************
	Problem: 1003
	User: hbq
	Language: C++
	Result: Accepted
	Time:15 ms
	Memory:2072 kb
****************************************************************/