import java.io.{BufferedOutputStream, BufferedReader, InputStreamReader, PrintWriter}

object Main {
  def main(args: Array[String]): Unit = {
    val in = new BufferedReader(new InputStreamReader(System.in))
    val out = new PrintWriter(new BufferedOutputStream(System.out))
    val Array(n, m, d) = in.readLine().split("\\s+").map(_.toInt)

    if (d == 0) {
      out.println(f"${1.0 / n * (m - 1)}%1.10f")
    } else {
      out.println(f"${2.0 * (n - d) / n / n * (m - 1)}%1.10f")
    }
    out.flush()
  }
}
