import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Arrays;
import java.io.BufferedWriter;
import java.util.Collection;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.UncheckedIOException;
import java.nio.charset.Charset;
import java.util.StringTokenizer;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.Queue;
import java.io.BufferedReader;
import java.util.ArrayDeque;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author mikit
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        LightScanner in = new LightScanner(inputStream);
        LightWriter out = new LightWriter(outputStream);
        ADarkerAndDarker solver = new ADarkerAndDarker();
        solver.solve(1, in, out);
        out.close();
    }

    static class ADarkerAndDarker {
        public void solve(int testNumber, LightScanner in, LightWriter out) {
            // out.setBoolLabel(LightWriter.BoolLabel.YES_NO_FIRST_UP);
            int h = in.ints(), w = in.ints();
            int[][] res = new int[h][w];
            ArrayUtil.fill(res, 20000);

            Queue<Vec2i> q = new ArrayDeque<>();
            for (int i = 0; i < h; i++) {
                String s = in.string();
                for (int j = 0; j < w; j++) {
                    if (s.charAt(j) == '#') {
                        res[i][j] = 0;
                        q.add(new Vec2i(j, i));
                    }
                }
            }
            while (!q.isEmpty()) {
                Vec2i v = q.poll();
                GeoWalker.forEach4i(v.x, v.y, (nx, ny) -> {
                    if (nx < 0 || ny < 0 || nx >= w || ny >= h) return;
                    if (res[v.y][v.x] + 1 >= res[ny][nx]) return;
                    res[ny][nx] = res[v.y][v.x] + 1;
                    q.offer(new Vec2i(nx, ny));
                });
            }

            int ans = 0;
            for (int i = 0; i < h; i++) {
                for (int j = 0; j < w; j++) {
                    ans = Math.max(ans, res[i][j]);
                }
            }
            out.ans(ans).ln();
        }

    }

    static interface BiIntConsumer {
        void accept(int t, int u);

    }

    static class Vec2i implements Comparable<Vec2i> {
        public int x;
        public int y;

        public Vec2i(int x, int y) {
            this.x = x;
            this.y = y;
        }

        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Vec2i vec2i = (Vec2i) o;
            return x == vec2i.x && y == vec2i.y;
        }

        public int hashCode() {
            return 31 * x + y;
        }

        public String toString() {
            return "(" + x + ", " + y + ")";
        }

        public int compareTo(Vec2i o) {
            if (x == o.x) {
                return Integer.compare(y, o.y);
            }
            return Integer.compare(x, o.x);
        }

    }

    static class LightWriter implements AutoCloseable {
        private final Writer out;
        private boolean autoflush = false;
        private boolean breaked = true;

        public LightWriter(Writer out) {
            this.out = out;
        }

        public LightWriter(OutputStream out) {
            this(new BufferedWriter(new OutputStreamWriter(out, Charset.defaultCharset())));
        }

        public LightWriter print(char c) {
            try {
                out.write(c);
                breaked = false;
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
            return this;
        }

        public LightWriter print(String s) {
            try {
                out.write(s, 0, s.length());
                breaked = false;
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
            return this;
        }

        public LightWriter ans(String s) {
            if (!breaked) {
                print(' ');
            }
            return print(s);
        }

        public LightWriter ans(int i) {
            return ans(Integer.toString(i));
        }

        public LightWriter ln() {
            print(System.lineSeparator());
            breaked = true;
            if (autoflush) {
                try {
                    out.flush();
                } catch (IOException ex) {
                    throw new UncheckedIOException(ex);
                }
            }
            return this;
        }

        public void close() {
            try {
                out.close();
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
        }

    }

    static final class ArrayUtil {
        private ArrayUtil() {
        }

        public static void fill(int[] a, int v) {
            Arrays.fill(a, v);
        }

        public static void fill(int[][] a, int v) {
            for (int i = 0; i < a.length; i++) {
                fill(a[i], v);
            }
        }

    }

    static class LightScanner {
        private BufferedReader reader = null;
        private StringTokenizer tokenizer = null;

        public LightScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
        }

        public String string() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new UncheckedIOException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int ints() {
            return Integer.parseInt(string());
        }

    }

    static final class GeoWalker {
        private static final int[] DX = {1, 0, -1, 0, 1, 1, -1, -1};
        private static final int[] DY = {0, 1, 0, -1, 1, -1, -1, 1};

        private GeoWalker() {
        }

        public static void forEach4i(int x, int y, BiIntConsumer receiver) {
            for (int i = 0; i < 4; i++) {
                receiver.accept(x + DX[i], y + DY[i]);
            }
        }

    }
}



Measured memory_bytes: 69860