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

/**************************************************************
	Problem: 1003
	User: chenjingqi
	Language: C++
	Result: Accepted
	Time:18 ms
	Memory:2072 kb
****************************************************************/