#include<bits/stdc++.h>
using namespace std;
int main(){
	int a=0,s=0;
	cin>>a;
	while(a>=1){
		s=s+a;
		a=a-2;
	}cout<<s;
}
/**************************************************************
	Problem: 1110
	User: tonghuawei
	Language: C++
	Result: Wrong Answer
****************************************************************/