#include<bits/stdc++.h>
using namespace std;
int n,s=1,h;
int main(){
	cin>>n;
    while(n>=s){
    	h=h+s;
    	s=s+2;
    }
    cout<<h;
}
    
/**************************************************************
	Problem: 1003
	User: chenyichuen
	Language: C++
	Result: Accepted
	Time:19 ms
	Memory:2072 kb
****************************************************************/