#include<bits/stdc++.h>
using namespace std;

int main(void){
    
    int n,sum=0;
    cin>>n;
    
    for(int i=1 ; i<=n ; i+=2){
        sum+=i;
    }
    
    cout<<sum<<endl;
    return 0;
}
/**************************************************************
	Problem: 1003
	User: 231308061
	Language: C++
	Result: Accepted
	Time:10 ms
	Memory:2072 kb
****************************************************************/