#include<bits/stdc++.h>
using namespace std;
int main(){
    int n,s=1,h=0;
    cin>>n;
    while(s<=n){
    h=h+s;
    s+=2;
        }
    cout<<h;
 
 
 
     
}
/**************************************************************
	Problem: 1003
	User: chenmingyu
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/