#include <iostream>
#include <algorithm>
#include <cstdio>
#include <string>

#define ll long long

using namespace std;


int main(){
    
    int L;
    cin>>L;
    ll ans=0;
    for(int i=L-2;i>=1;i-=2) ans+=i;

    cout<<ans<<endl;

    return 0;
}