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

/**************************************************************
	Problem: 1003
	User: wangyichen2
	Language: C++
	Result: Compile Error
****************************************************************/