import java.util.Scanner

class No521() {
	var input = Scanner(System. `in`)
	val boxes = input.nextInt()
	val danger = input.nextInt()
	var boxList = Array<Boolean>(boxes, {false})

	fun dengerous(): Array<Boolean> {
		if (0 < danger && danger <= boxes) {
			boxList[danger - 1] = true
			boxList[boxList.size - danger] = true
		}
		return boxList
	}
}

fun main(args: Array<String>) {
	val no521 = No521()
	val safety = no521.dengerous().filter{i -> i == false}.size
	if (safety == no521.boxes) println(0)
	else println(safety)
}