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

/**************************************************************
	Problem: 1110
	User: lijinkai
	Language: C++
	Result: Time Limit Exceed
****************************************************************/