import kotlin.math.hypot

fun main() {
    val (x, y, z, w) = readLine()!!.split(" ").map { it.toDouble() }

    var l = 0.0; var r = x;
    for (i in 0 until 100) {
        val ab = (l+r) / 2
        val ad = x / ab

        if (hypot(ab, y) - hypot(ad, z) <= w) l = ab
        else r = ab
    }

    val ab = l
    val ad = x / ab
    val abp = ab * y / 2
    val dpa = (ad-y) * (ab-z) / 2
    val bcq = ad * z / 2

    println(x - abp - dpa - bcq)
}