#include<iostream> 

using namespace std;

int main()
{
	
	int sum=0,n;
	cin>>n;
	
	for(int i=1 ; i<=n ; i+=2){
		sum+=i;
	}
	
	cout<<sum<<endl;
	return 0;
}
/**************************************************************
	Problem: 1003
	User: 231308061
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/