#include <iostream>
#include <iomanip>
#include <sstream>
#include <vector>
#include <string>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <functional>
#include <iterator>
#include <limits>
#include <numeric>
#include <utility>
#include <cmath>
#include <cassert>
#include <cstdio>

using namespace std; using namespace placeholders;

using LL = long long;
using ULL = unsigned long long;
using VI = vector< int >;
using VVI = vector< vector< int > >;
using VS = vector< string >;
using SS = stringstream;
using PII = pair< int, int >;
using VPII = vector< pair< int, int > >;
template < typename T = int > using VT = vector< T >;
template < typename T = int > using VVT = vector< vector< T > >;
template < typename T = int > using LIM = numeric_limits< T >;

template < typename T > inline istream& operator>>( istream &s, vector< T > &v ){ for ( T &t : v ) { s >> t; } return s; }
template < typename T > inline ostream& operator<<( ostream &s, const vector< T > &v ){ for ( int i = 0; i < int( v.size() ); ++i ){ s << ( " " + !i ) << v[i]; } return s; }
template < typename T > inline T fromString( const string &s ) { T res; istringstream iss( s ); iss >> res; return res; };
template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); };

#define REP2( i, n ) REP3( i, 0, n )
#define REP3( i, m, n ) for ( int i = ( int )( m ); i < ( int )( n ); ++i )
#define GET_REP( a, b, c, F, ... ) F
#define REP( ... ) GET_REP( __VA_ARGS__, REP3, REP2 )( __VA_ARGS__ )
#define FOR( e, c ) for ( auto &e : c )
#define ALL( c ) ( c ).begin(), ( c ).end()
#define AALL( a, t ) ( t* )a, ( t* )a + sizeof( a ) / sizeof( t )
#define DRANGE( c, p ) ( c ).begin(), ( c ).begin() + p, ( c ).end()

#define SZ( v ) ( (int)( v ).size() )
#define PB push_back
#define EM emplace
#define EB emplace_back
#define BI back_inserter

#define EXIST( c, e ) ( ( c ).find( e ) != ( c ).end() )

#define MP make_pair
#define fst first
#define snd second

#define DUMP( x ) cerr << #x << " = " << ( x ) << endl

constexpr int INF = LIM<>::max() / 2;
constexpr int dy[] = { 0, 1, 0, -1 };
constexpr int dx[] = { 1, 0, -1, 0 };

int main()
{
	cin.tie( 0 );
	ios::sync_with_stdio( false );

	int W, H;
	cin >> W >> H;

	VS board( H );
	cin >> board;

	VVI distances( H, VI( W, INF ) );
	queue< PII > que;
	REP( i, H )
	{
		for ( int j = 0; j < W && que.empty(); ++j )
		{
			if ( board[i][j] == '.' )
			{
				distances[i][j] = 0;
				que.EM( i, j );
			}
		}
	}
	while ( !que.empty() )
	{
		const int y = que.front().fst;
		const int x = que.front().snd;
		que.pop();

		REP( d, 4 )
		{
			const int ny = y + dy[d];
			const int nx = x + dx[d];

			if ( 0 <= ny && ny < H && 0 <= nx && nx < W && board[ ny ][ nx ] == '.' && distances[ ny ][ nx ] != 0 )
			{
				distances[ ny ][ nx ] = 0;
				que.EM( ny, nx );
			}
		}
	}
	REP( i, H )
	{
		REP( j, W )
		{
			if ( distances[i][j] == 0 )
			{
				que.EM( i, j );
			}
		}
	}
	while ( !que.empty() )
	{
		const int y = que.front().fst;
		const int x = que.front().snd;
		que.pop();

		REP( d, 4 )
		{
			const int ny = y + dy[d];
			const int nx = x + dx[d];

			if ( 0 <= ny && ny < H && 0 <= nx && nx < W && distances[y][x] + 1 < distances[ ny ][ nx ] )
			{
				distances[ ny ][ nx ] = distances[y][x] + 1;
				que.EM( ny, nx );
			}
		}
	}

	int res = INF;
	REP( i, H )
	{
		REP( j, W )
		{
			if ( board[i][j] == '.' && distances[i][j] )
			{
				res = min( res, distances[i][j] - 1 );
			}
		}
	}
	cout << res << endl;

	return 0;
}