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