private fun readStrings() = readLine()!!.split(" ")
private fun readInts() = readStrings().map { it.toInt() }

fun main(args: Array<String>) {
    val (n, m, d) = readInts()
    val diff = if (d == 0) n else (n - d) * 2
    val answer = (diff.toDouble() / n) * ((m - 1).toDouble() / n)
    println("%.9f".format(answer))
}