#include<bits/stdc++.h>
using namespace std;
int main(){
    int n,a=0;
    cin>>n;
    for(int i=1;i<=n;i++){
        a+=i;
        i++;
    }
    cout<<a;
    return 0;
}
/**************************************************************
	Problem: 1003
	User: jiangyuanfeng
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2072 kb
****************************************************************/