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

fun main(args: Array<String>) {
    val (N, K) = readIntList()
    val A = readIntList()

    var count = K
    var times = 1
    while (count < N) {
        count += (K - 1)
        times++
    }

    println(times)
}