import java.util.*

fun main(vararg args: String) {
    val scanner = Scanner(System.`in`)
    val sideList = mutableListOf<Int>()
    sideList.add(scanner.nextInt())
    sideList.add(scanner.nextInt())
    sideList.add(scanner.nextInt())
    val triangle = Triangle(sideList.sortedBy { it })
    println(triangle.calculateArea())
}

class Triangle constructor(private val sides: List<Int>) {
    fun calculateArea(): Int {
        return sides[0] * sides[1] / 2
    }
}