import java.util.Scanner

class No521() {
	var input = Scanner(System. `in`)
	val boxes = input.nextInt()
	val danger = input.nextInt()

	fun dengerous() {
		if (danger == 0 || danger > boxes) println(0)
		else {
			if (boxes % 2 == 1 && danger == Math.round(boxes / 2.0).toInt()) println(boxes - 1)
			else println(boxes - 2)
		}
	}
}

fun main(args: Array<String>) {
	val no521 = No521()
	no521.dengerous()
}