import scala.math

object Main {
	def main(args: Array[String]) {
		val input = io.Source.stdin.getLines().mkString("\n")
		println(solve(input).trim)
	}

	def solve(input: String): String = {
		 val a = input.toInt
		 val ans = a * 2 * math.Pi
		 ans.toString
	}

}