fun main() {
    val (a, b, c, d) = readInts().map { it.toLong() }
    val poss = listOf(a * c, a * d, b * c, b * d)
    println(poss.max())
}

// Input Reader
private fun readLn() = readLine()!!

private fun readStrings() = readLn().trim().split(" ")
private fun readInts() = readStrings().map { it.toInt() }
