import java.io._
import java.util.StringTokenizer
import scala.util.control.Breaks.{breakable,break}

object Main extends App {
  val in = new InputReader(System.in)
  val pw = new PrintWriter(System.out)

  val n, m, d = in.next().toLong

  val oneBlock = {
    if(d > (n/2)) {
      if(d % 2 == 0) {
        val zero = (d - (n/2)) * 2
        val one = n - zero

        one / (n*n)
      }
      else{
        val zero = (d - (n/2)) * 2 - 1
        val one = n - zero

        one / (n*n)
      }
    }
    else if(d == 0) {
      n.toDouble / (n*n)
    }
    else {
      (d*2 + (n - 2*d)*2L).toDouble / (n*n)
    }
  }


  println(oneBlock * (m-1))
}


class InputReader(stream: InputStream) {
  private val reader = new BufferedReader(new InputStreamReader(stream))
  private var tokenizer: StringTokenizer = new StringTokenizer(reader.readLine())
  def next(): String = {
    while (!tokenizer.hasMoreTokens()) {
      tokenizer = new StringTokenizer(reader.readLine())
    }
    tokenizer.nextToken()
  }
}

