#include<bits/stdc++.h>
using namespace std;
int n,i=1,s;
int main(){
	cin>>n;
	while(i<=n){
		s=s+i;
		i=i+2;
	}
	cout<<s;
	return 0;
}
/**************************************************************
	Problem: 1110
	User: zhangziang
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/