import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        
        long l = sc.nextLong();
        
        long ans = 0l;
        for (long i = 1; i <= l - 2; i = i + 2) {
            ans += i;
        }
        
        System.out.println(ans);
        
    }
}