/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author Ido Kessler
 */

#pragma GCC optimize("O3")

#include <iostream>

#include <vector>
#include <queue>
#include <sstream>
#include <algorithm>
#include <numeric>
#include <functional>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <bitset>
#include <iomanip>
#include <cstdint>
#include <cassert>
#include <complex>

#include <tuple>

#define all(x) (x).begin(), (x).end()
#define fori(i, n) for(int i = 0; i < n; i++)
#define rep(i, s, e) for(int i = (s); i <= (e); ++i)
#define repr(i, s, e) for(int i = (s); i >= (e); --i)
#define ll long long int
#define pb push_back

namespace io{
class input_reader{
std::istream&in;
void throw_error(){throw std::runtime_error("No more inputs while reading number");}
template<typename T>
inline void readIntegerValue(T&f){bool positive=true;f=0;char c;
if(!in.get(c))throw_error();
while(c<'0'||c>'9'){if(c=='-')positive=false;if(!in.get(c))throw_error();}
while(c>='0'&&c<='9'){f=(f<<3)+(f<<1)+(c&15);if(!in.get(c))break;}
if(!positive)f*=-1;}
public:
explicit input_reader(std::istream&in):in(in){}
inline void read(int&f){readIntegerValue(f);}
inline void read(short&f){readIntegerValue(f);}
inline void read(long int&f){readIntegerValue(f);}
inline void read(long long int&f){readIntegerValue(f);}
inline void read(unsigned int&f){readIntegerValue(f);}
inline void read(unsigned short&f){readIntegerValue(f);}
inline void read(unsigned long int&f){readIntegerValue(f);}
inline void read(unsigned long long int&f){readIntegerValue(f);}
template<typename T>inline void read(std::vector<T>&f){for(auto&i:f)read(i);}
template<typename T,int N>inline void read(std::array<T,N>&res){for(auto&i:res)read(i);}
template<typename T1,typename T2>inline void read(std::pair<T1,T2>&p){in.read(p.first,p.second);}
template<typename T>inline void read(T&f){in>>f;}
template<typename T,typename...Args>inline void read(T&t,Args&...args){read(t),read(args...);}
template<typename T>inline void readArray(int n,T*res){while(n--)read(*(res++));}
template<typename T>inline input_reader&operator>>(T&t){return read(t),*this;}
};
}

namespace io{
class output_writer{
std::ostream&out;
template<typename T>inline void printIntegerHelper(T x){if(x>=10)printIntegerHelper(x/10);out.put(x%10+48);}
template<typename T>inline void printInteger(T x){if(x<0)out.put('-'),x=-x;printIntegerHelper(x);}
template<typename T>inline void printArray(T&ar){bool first=true;for(auto&t:ar)print((first)? first=false,"":" ",t);}
public:
explicit output_writer(std::ostream&out):out(out){}
inline void flush(){out.flush();}
inline void print(){}
inline void print(char x){out.put(x);}
inline void print(short x){printInteger(x);}
inline void print(int x){printInteger(x);}
inline void print(long int x){printInteger(x);}
inline void print(long long int x){printInteger(x);}
inline void print(unsigned short x){printInteger(x);}
inline void print(unsigned int x){printInteger(x);}
inline void print(unsigned long int x){printInteger(x);}
inline void print(unsigned long long int x){printInteger(x);}
template<typename T1>inline void print(const T1&t1){out<<t1;}
template<typename T1>inline void print(const std::vector<T1>&t1){printArray(t1);}
template<typename T1,int N>inline void print(const std::array<T1,N>&t1){printArray(t1);}
template<typename T1,int N,int M>inline void print(const std::array<std::array<T1,M>,N>&t1){for(auto&t:t1)println(t);}
template<typename T1>inline void print(const std::vector<std::vector<T1>>&t1){for(auto&t:t1)println(t);}
template<typename T,typename...Args>inline void print(const T&t,Args...args){print(t),print(args...);}
template<typename...Args>inline void println(Args...args){print(args...),print('\n');}
template<typename T>output_writer&operator<<(const T&t){return print(t),*this;}
};
}

inline ll pow_mod(ll a,ll b,const ll MOD){ll res=1;for(ll i=1;b;b^=b&i,i<<=1,a=(a*a)%MOD)if(b&i)res=(res*a)%MOD;return res;}
inline ll mod_inv(const ll a,const ll MOD){return pow_mod(a,MOD-2,MOD);}
template<typename T>inline T gcd(T a,T b){T t;while(a!=0)t=a,a=b%a,b=t;return b;}
template<typename T>T lcm(const T &a,const T &b) {assert(a!=0&&b!=0);return a/gcd(a,b)*b;}




using namespace std;

class ADarkerAndDarker {
	class Solver {
		io::input_reader in;
		io::output_writer out;

	public:
		Solver(std::istream &in, std::ostream &out) : in(in), out(out) {}

		void solve() {
			int h, w;
			in.read(h, w);
			int hw[h][w];
			fori(i, h) fori(j, w) hw[i][j] = 1000000000;
			fori(i, h) {
				string s;
				in >> s;
				fori(j, w) if (s[j] == '#') hw[i][j] = 0;
			}
			vector<pair<int, int>> q[2];
			int p = 0;

			fori(i, h) fori(j, w) if (hw[i][j] == 0) q[0].pb({i, j});
			int dist = 0;
			while (q[p].empty() == false) {
				for (auto pi : q[p]) {
					rep(i1, -1, 1) {
						rep(j1, -1, 1) {
							int i = i1 + pi.first;
							int j = j1 + pi.second;
							if (i1 != 0 && j1 != 0) continue;
							if (i < 0 || j < 0)continue;
							if (i >= h || j >= w) continue;
							if (hw[i][j] <= dist + 1) continue;
							hw[i][j] = dist + 1;
							q[p ^ 1].pb({i, j});
						}
					}
				}
				q[p].clear();
				p ^= 1;
				dist++;
			}
			int res = 0;
			fori(i, h) fori(j, w) res = max(res, hw[i][j]);
			out << res;
		}
	};

public:
	void solve(std::istream &in, std::ostream &out) {
		Solver(in, out).solve();
	}
};

int main() {
	std::ios_base::sync_with_stdio(false), cin.tie(0);
	ADarkerAndDarker solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}


Measured memory_bytes: 18152