import java.lang.Math.sqrt

fun area(a: Double, b: Double, c: Double): Double {
    val s = (a + b + c) / 2
    return sqrt(s * (s - a) * (s - b) * (s - c))
}
fun main(args: Array<String>?): Unit {
    while (true) {
        readLine()?.let {
            val (a, l, x) = it.split(' ').map(String::toDouble)
            println(String.format("%.15f", area(a, l, l) + area(l, (l + x) / 2, (l + x) / 2) * 2))
        } ?: break
    }
}
