import java.io.BufferedReader
import java.io.InputStream
import java.io.InputStreamReader
import java.io.PrintWriter
import java.util.*
import kotlin.collections.ArrayList

fun PrintWriter.solve(sc: FastScanner) {
    val n = sc.nextInt()
    val m = sc.nextInt()
    val c = sc.nextLong()
    val a = Array(n) { sc.nextLong() }
    val b = Array(m) { sc.nextLong() }
    var total = 0L
    a.sort()
    b.sort()
    var j = m - 1
    for (i in 0 until n) {
        while (j > 0 && a[i] * b[j - 1] > c) j--
        if (a[i] * b[j] > c) {
            total += m - j
        }
    }
    println(total / (n * m).toDouble())
}

fun main() {
    val writer = PrintWriter(System.out, false)
    writer.solve(FastScanner(System.`in`))
    writer.flush()
}

class FastScanner(s: InputStream) {
    private var st = StringTokenizer("")
    private val br = BufferedReader(InputStreamReader(s))

    fun next(): String {
        while (!st.hasMoreTokens()) st = StringTokenizer(br.readLine())

        return st.nextToken()
    }

    fun nextInt() = next().toInt()
    fun nextLong() = next().toLong()
    fun nextLine() = br.readLine()
    fun nextDouble() = next().toDouble()
    fun ready() = br.ready()
}