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

/**************************************************************
	Problem: 1003
	User: zzz1
	Language: C
	Result: Compile Error
****************************************************************/