import java.util.Scanner

//
fun _Do() {
    
    val N = scan.nextInt()
    val K = scan.nextInt()
    
    println( when{
        K==0 || K>N -> 0
        K*2 == N+1 -> N-1
        else -> N-2
    }
    )
}

var scan = Scanner(System.`in`)
fun main( argv : Array<String> ) {
    _Do()
}