package main

import . "fmt"

func main() {
	var n,k int
	Scan(&n,&k)
	switch {
		default:
			Println(n-2)
		case n%2==1 && (n+1)/2==k:
			Println(n-1)
		case k == 0 || k > n:
			Println(0)
	}
}