#include <iostream>

using namespace std;

int main(){
    int n,i,s=0;
    cin>>n;
    for(i=1;i<=n;i+=2){
        s+=i;
    }
    cout<<s<<endl;
}
/**************************************************************
	Problem: 1110
	User: admin
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/