import java.util.*;
import java.math.BigDecimal;

public class Main {
    public static void main(final String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int L = sc.nextInt();
        sc.close();
        double syo = L / 3.0;
        double amari = L - syo * 3.0;
        double V;
        if(amari > 0)
            V = syo * syo * (syo + amari);
        else
            V = syo * syo * syo;

        System.out.println(BigDecimal.valueOf(V).toPlainString());
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz