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

/**************************************************************
	Problem: 1110
	User: chenzelin
	Language: C++
	Result: Accepted
	Time:15 ms
	Memory:2072 kb
****************************************************************/