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

/**************************************************************
	Problem: 1003
	User: linweilun
	Language: C++
	Result: Accepted
	Time:10 ms
	Memory:2072 kb
****************************************************************/