import java.util.*

fun main(args: Array<String>) {
    agc033a()
}

private data class Coordinate(val y: Int, val x: Int)

fun agc033a() {
    val (h, w) = readLine()!!.split(' ').map { it.toInt() }
    val isBlack = (1..h).map { readLine()!!.map { it == '#' }.toBooleanArray() }.toTypedArray()

    val blackIndices = mutableListOf<Coordinate>()
    val distantList = Array(h) { IntArray(w) { 99999 } }

    for (y in 0 until h) {
        for (x in 0 until w) {
            if (isBlack[y][x]) {
                blackIndices.add(Coordinate(y, x))
                distantList[y][x] = 0
            }
        }
    }

    val queue = ArrayDeque(blackIndices)
    val direction = listOf(-1 to 0, 1 to 0, 0 to -1, 0 to 1)
    var answer = 0

    while (queue.isNotEmpty()) {
        val (y, x) = queue.poll()

        val dist = distantList[y][x]
        answer = Math.max(answer, dist)

        direction.forEach {
            val nextY = y + it.first
            val nextX = x + it.second
            if (nextY in 0 until h && nextX in 0 until w && !isBlack[nextY][nextX]) {
                isBlack[nextY][nextX] = true
                queue.add(Coordinate(nextY, nextX))
                distantList[nextY][nextX] = Math.min(distantList[nextY][nextX], dist + 1)
            }
        }
    }

    println(answer)
}
