#include<bits/stdc++.h>
using namespace std;
int main()
{

int n;
cin>>n;
int i=1,sum=0;
while(i<=n){
	if(i%2!=0){	
		sum+=i; 
	}
	i++;
}
cout<<sum<<endl;



	return 0;
}

/**************************************************************
	Problem: 1002
	User: hbq
	Language: C++
	Result: Wrong Answer
****************************************************************/