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

/**************************************************************
	Problem: 1003
	User: duyahan
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/