import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static string[] s_rd;
T RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
string RDR()() { return readln.chomp; }
T[] ARR(T = long)(in string str, T fix = 0) { auto r = str.split.to!(T[]); r[] += fix; return r; }
size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
bool minimize(T)(ref T x, T y) { if (x > y) { x = y; return true; } else { return false; } }
bool maximize(T)(ref T x, T y) { if (x < y) { x = y; return true; } else { return false; } }
long lcm(long x, long y) { return x * y / gcd(x, y); }

long mod = 10^^9 + 7;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }

void main()
{
	auto H = RD;
	auto W = RD;
	auto A = new string[](H);
	foreach (i; 0..H)
	{
		A[i] = RD!string;
	}
	
	long[3][] open;
	auto used = new long[][](H, W);
	foreach (i; 0..H)
	{
		fill(used[i], long.max);
		foreach (j; 0..W)
		{
			if (A[i][j] == '#')
			{
				open ~= [i, j, 0];
				used[i][j] = 0;
			}
		}
	}

	long ans;
	void f(long y, long x, long c)
	{
		if (c < used[y][x])
		{
			used[y][x] = c;
			open ~= [y, x, c];
			ans = max(ans, c);
		}
	}
	while (!open.empty)
	{
		auto n = open.front; open.popFront;
		auto y = n[0];
		auto x = n[1];
		auto c = n[2] + 1;
		if (inside(y-1, 0, H))
		{
			f(y-1, x, c);
		}
		if (inside(y+1, 0, H))
		{
			f(y+1, x, c);
		}
		if (inside(x-1, 0, W))
		{
			f(y, x-1, c);
		}
		if (inside(x+1, 0, W))
		{
			f(y, x+1, c);
		}
	}

	writeln(ans);
	stdout.flush();
	debug readln();
}