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

}

/**************************************************************
	Problem: 1003
	User: mc002
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2072 kb
****************************************************************/