import java.util.*
import java.util.NoSuchElementException
import java.io.IOException
import java.util.PriorityQueue
import java.util.Arrays
import java.util.ArrayList



var sc : FastScanner = FastScanner()

fun main(args: Array<String>) {
    var N = sc.nextInt()
    var K = sc.nextInt()

    if(N == 0 || K > N) println("0")
    else {
        if(N % 2 == 1 && (N + 1) / 2 == K) println((N - 1).toString())
        else println((N - 2).toString())
    }
}





class FastScanner {
    private val `in` = System.`in`
    private val buffer = ByteArray(1024)
    private var ptr = 0
    private var bufferLength = 0

    private fun hasNextByte(): Boolean {
        if (ptr < bufferLength) {
            return true
        } else {
            ptr = 0
            try {
                bufferLength = `in`.read(buffer)
            } catch (e: IOException) {
                e.printStackTrace()
            }

            if (bufferLength <= 0) {
                return false
            }
        }
        return true
    }

    private fun readByte(): Int {
        if (hasNextByte())
            return buffer[ptr++].toInt()
        else
            return -1
    }

    private fun isPrintableChar(c: Int): Boolean {
        return 33 <= c && c <= 126
    }

    private fun skipUnprintable() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr].toInt())) ptr++
    }

    internal operator fun hasNext(): Boolean {
        skipUnprintable()
        return hasNextByte()
    }

    operator fun next(): String {
        if (!hasNext()) throw NoSuchElementException()
        val sb = StringBuilder()
        var b = readByte()
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b)
            b = readByte()
        }
        return sb.toString()
    }

    internal fun nextLong(): Long {
        if (!hasNext()) throw NoSuchElementException()
        var n: Long = 0
        var minus = false
        var b = readByte()
        if (b == '-'.toInt()) {
            minus = true
            b = readByte()
        }
        if (b < '0'.toInt() || '9'.toInt() < b) {
            throw NumberFormatException()
        }
        while (true) {
            if ('0'.toInt() <= b && b <= '9'.toInt()) {
                n *= 10
                n += (b - '0'.toInt()).toLong()
            } else if (b == -1 || !isPrintableChar(b)) {
                return if (minus) -n else n
            } else {
                throw NumberFormatException()
            }
            b = readByte()
        }
    }

    internal fun nextDouble(): Double {
        return java.lang.Double.parseDouble(next())
    }

    internal fun nextDoubleArray(n: Int): DoubleArray {
        val array = DoubleArray(n)
        for (i in 0..n - 1) {
            array[i] = nextDouble()
        }
        return array
    }

    internal fun nextDoubleMap(n: Int, m: Int): Array<DoubleArray?> {
        val map = arrayOfNulls<DoubleArray?>(n)
        for (i in 0..n - 1) {
            map[i] = nextDoubleArray(m)
        }
        return map
    }

    fun nextInt(): Int {
        return nextLong().toInt()
    }

    fun nextIntArray(n: Int): IntArray {
        val array = IntArray(n)
        for (i in 0..n - 1) {
            array[i] = nextInt()
        }
        return array
    }
}