#include <bits/stdc++.h>

using namespace std;

int main(){
    int n;
    cin>>n;
    int i,c=0;
    for(i=1;i<=n;i=i+2){
        c=c+i;	
	}cout<<c<<endl;
    return 0;
}
/**************************************************************
	Problem: 1003
	User: zhuangsongyu
	Language: C++
	Result: Accepted
	Time:14 ms
	Memory:2072 kb
****************************************************************/