import java.io.*;
import java.io.IOException;
import java.util.*;
import java.text.DecimalFormat;

class Main {
	public static long mod = (long)Math.pow(10, 9)+7 ;
	public static double epsilon=0.00000000008854;//value of epsilon
	public static InputReader sc = new InputReader(System.in);
	public static PrintWriter pw = new PrintWriter(System.out);
	public static long gcd(long a,long b){
		while(b>0){
			long t=b;
			b=a%b;
			a=t;
		}
		return a;
    }
    public static long lcm(long a,long b){
        return (a*b)*modInverse(gcd(a, b), mod);
    }
    
    public static long LcmArray(long a[]){
		long ans=1;
		for(int i=0;i<a.length;i++){
            ans=lcm(ans,a[i]);
            ans%=mod;
		}
		return ans;
    }
	
	
	public static long bSearch(Pair[]d, int l, int r, int a) {
		if(d[l].d > a)
			return 0;
		if(l == r)
			return d[l].g;
		int m = (l + r + 1) / 2;
		if(d[m].d > a)
			return bSearch(d, l, m - 1, a);
		return bSearch(d, m, r, a);
	}
	static class Pair implements Comparable<Pair> {
		int d, i;
		long g;
		public Pair(int D, long G, int I) {
			d = D;
			g = G;
			i = I;
		}
		public int compareTo(Pair p) {
			if(d == p.d)
				return i - p.i;
			return d - p.d;
		}
	 }
	 
	 public static void dfs(int s,int a[][],boolean vis[],boolean tvis[]){
		int n=a.length;
		for(int i=0;i<n;i++){
			if(a[s][i]==1&&!vis[i]){
				vis[i]=true;
				if(tvis[i]){
					for(int j=0;j<n;j++){
						if(a[i][j]==1)
						vis[j]=true;
					}
				}
				else
				dfs(i, a, vis,tvis);
			}
		}
	 }
	
	public static int eq(int a,int b){
		if(a==b)
		return 1;
		else
		return 0;
	}

    public static long modInverse(long q, long mod2) {
		return modularExponentiation(q, mod2 - 2, mod2);
	}
    public static long modularExponentiation(long x, long n, long M) {
		long result = 1;
		while (n > 0) {
			if (n % 2 == 1)
				result = (result * x) % M;
			x = (x * x) % M;
			n = n / 2;
		}
		return result;
	}
	public static void main(String[] args) {
		// code starts..
        //Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		double l=(double)n/3.0;
		Double ans=Math.pow(l, 3);
		pw.println(ans.toString());



		// Code ends...
		//sc.close();
		pw.flush();
		pw.close();
	}
	static class tripletL implements Comparable<tripletL> {
		Long x, y, z;
 
		tripletL(long x, long y, long z) {
			this.x = x;
			this.y = y;
			this.z = z;
		}
 
		public int compareTo(tripletL o) {
			int result = x.compareTo(o.x);
			if (result == 0)
				result = y.compareTo(o.y);
			if (result == 0)
				result = z.compareTo(o.z);
 
			return result;
		}
 
		public boolean equlas(Object o) {
			if (o instanceof tripletL) {
				tripletL p = (tripletL) o;
				return (x - p.x == 0) && (y - p.y ==0 ) && (z - p.z == 0);
			}
			return false;
		}
 
		public String toString() {
			return x + " " + y + " " + z;
		}
 
		public int hashCode() {
			return new Long(x).hashCode() * 31 + new Long(y).hashCode() + new Long(z).hashCode();
		}
	}
	public static String Doubleformate(double a,int n){
		String s="";
		while(n-->0){
			s+='0';
		}
		DecimalFormat f =new DecimalFormat("#0."+s);
		return f.format(a);
	}
 
	public static Comparator<Integer[]> column(int i){
		return 
		new Comparator<Integer[]>() {
			@Override
			public int compare(Integer[] o1, Integer[] o2) {
				//return o1[i].compareTo(o2[i]);//for ascending
				return o2[i].compareTo(o1[i]);//for descending
			}
		};
	}
	public static Comparator<Integer[]> pair(){
		return 
		new Comparator<Integer[]>() {
			@Override
			public int compare(Integer[] o1, Integer[] o2) {
				int result=o1[0].compareTo(o2[0]);
				if(result==0)
				result=o1[1].compareTo(o2[1]);
				return result;
			}
		};
	}
	public static Comparator<Integer[]> Triplet(){
		return 
		new Comparator<Integer[]>() {
			@Override
			public int compare(Integer[] o1, Integer[] o2) {
			
					for(int i=0;i<3;i++){
						for(int j=i+1;j<3;j++){
							for(int k=0;k<3;k++){
								for(int p=k+1;p<3;p++){
									if((o1[i]==o2[k]&&o1[j]==o2[p])||(o1[j]==o2[k]&&o1[i]==o2[p])){
 
									}
								}
							}
						}
					}
					int result=o1[0].compareTo(o2[0]);
					if(result==0)
					result=o1[1].compareTo(o2[1]);
					if(result==0)
					result=o1[2].compareTo(o2[2]);
					return result;
			}
		};
	}
	
	
	public static String reverseString(String s){
		StringBuilder input1 = new StringBuilder(); 
        input1.append(s);  
		input1 = input1.reverse();
		return input1.toString();
	}
	public static int[] scanArray(int n){
		int a[]=new int [n];
		for(int i=0;i<n;i++)
		a[i]=sc.nextInt();
 
		return a;
	}
	public static long[] scanLongArray(int n){
		long a[]=new long [n];
		for(int i=0;i<n;i++)
		a[i]=sc.nextLong();
 
		return a;
	}
	public static String [] scanStrings(int n){
		String a[]=new String [n];
		for(int i=0;i<n;i++)
		a[i]=sc.nextLine();
 
		return a;
	}
	static class InputReader {
 
		private final InputStream stream;
		private final byte[] buf = new byte[8192];
		private int curChar, snumChars;
		private SpaceCharFilter filter;
 
		public InputReader(InputStream stream) {
			this.stream = stream;
		}
 
		public int snext() {
			if (snumChars == -1)
				throw new InputMismatchException();
			if (curChar >= snumChars) {
				curChar = 0;
				try {
					snumChars = stream.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (snumChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}
 
		public int nextInt() {
			int c = snext();
			while (isSpaceChar(c)) {
				c = snext();
			}
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = snext();
			}
			int res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public long nextLong() {
			int c = snext();
			while (isSpaceChar(c)) {
				c = snext();
			}
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = snext();
			}
			long res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public int[] nextIntArray(int n) {
			int a[] = new int[n];
			for (int i = 0; i < n; i++) {
				a[i] = nextInt();
			}
			return a;
		}
 
		public String readString() {
			int c = snext();
			while (isSpaceChar(c)) {
				c = snext();
			}
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = snext();
			} while (!isSpaceChar(c));
			return res.toString();
		}
 
		public String nextLine() {
			int c = snext();
			while (isSpaceChar(c))
				c = snext();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = snext();
			} while (!isEndOfLine(c));
			return res.toString();
		}
 
		public boolean isSpaceChar(int c) {
			if (filter != null)
				return filter.isSpaceChar(c);
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}
 
		private boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}
 
		public interface SpaceCharFilter {
			public boolean isSpaceChar(int ch);
		}
	}
 
}


Measured memory_bytes: 20692