import Foundation

/*
// 1
let N = Int(readLine()!)!

// 1 2
let split = readLine()!.componentsSeparatedByString(" ")
let N = Int(split[0])!
let S = Int(split[1])!

// 1 2 3 4 5
let array = readLine()!.componentsSeparatedByString(" ").map { Int($0)! }
*/

let N = Double(readLine()!)!
print((N / 3) * (N / 3) * (N / 3))