fun main(args: Array<String>){
    val n = readLine()!!.split(" ").map { it.toInt() }
    if (n[0] > 1){
        if (n[1] > 1){
            val a = n[0] -1
            val b = n[1] - 1
            val c = n[0] * a
            val d = c / 2
            val e = n[1] * b
            val f = e / 2
            val g = d + f
            println(g)
        } else {
            val a = n[0] -1
            val c = n[0] * a
            val d = c / 2
            println(d)
        }
    } else {
        if (n[1] > 1){
            val b = n[1] - 1
            val e = n[1] * b
            val f = e / 2
            println(f)
        } else {
            println(0)
        }
    }
}
