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

	return 0;
}

/**************************************************************
	Problem: 1003
	User: zy002
	Language: C++
	Result: Compile Error
****************************************************************/