fun main(){
  val x : Int = readInt()

  println(x * 3.141592 * 2)

}


fun readLn() = readLine()!! // string line
fun readInt() = readLn().toInt() // single int
fun readStrings() = readLn().split(" ") // list of strings
fun readInts() = readStrings().map { it.toInt() } // list of ints