package atcoder// TODO: ←消す

fun main(args: Array<String>) = ABC163A.main()

object ABC163A {

    fun main() {
        val R = readLine()!!.toDouble()
        println(2 * R * Math.PI)
    }


    /*####################################################################################*/

    fun readString() = readLine()!!
    fun readStrings() = readLine()!!.split(" ")
    fun readInt() = readLine()!!.toInt()
    fun readInts() = readLine()!!.split(" ").map { it.toInt() }
    fun readMatrixRows(height: Int) = (1..height).map { readLine()!!.toList() }

    fun Boolean.yesOrNo() = if (this) "Yes" else "No"
    fun Double.toNaturalExpr() = println("%f".format(this))
}