#include<iostream> 

using namespace std;

int main()
{
	
	int sum=0,n;
	cin>>n;
	int i=1
	while(i<=n){
		if(i%2!=0)sum+=i;
		i++;
	}
	
	cout<<sum<<endl;
	return 0;
}
/**************************************************************
	Problem: 1003
	User: 231308061
	Language: C++
	Result: Compile Error
****************************************************************/