import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
       int l = koko.nextInt();
       int n = (l-1)/2;
       int kai = 0;
       for(int i=0; i<n; i++){
           kai=kai+1+2*i;
       }
       System.out.println(kai);
    }
}