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

	
	


		
	return 0;
}
/**************************************************************
	Problem: 1003
	User: cyp
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/