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

/**************************************************************
	Problem: 1110
	User: huangcanwu
	Language: C++
	Result: Accepted
	Time:15 ms
	Memory:2072 kb
****************************************************************/