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

}
/**************************************************************
	Problem: 1003
	User: wtq001
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/