import java.util.*

fun main(args: Array<String>) {
    val sc = Scanner(System.`in`)
  	val a = sc.nextInt()
  	val b = sc.nextInt()
  	val c = sc.nextInt()
    var max = a
    if (b>max) max = b
    if (c>max) max = c
    println(a*b*c/max/2)
}
