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

/**************************************************************
	Problem: 1003
	User: lzc
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/