#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: 1003
	User: chenzelin
	Language: C++
	Result: Accepted
	Time:16 ms
	Memory:2072 kb
****************************************************************/