#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <queue>
#include <deque>
#include <bitset>
#include <iterator>
#include <list>
#include <stack>
#include <map>
#include <set>
#include <functional>
#include <numeric>
#include <utility>
#include <limits>
#include <time.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef vector<ll> vec;
typedef vector<vec> vec2;
typedef map<ll,ll> MPll;
typedef set<ll> setl;

const ll INF = 1ll << 60;
const ld EPS = 1e-10;
const int dx[4] = {1,0,-1,0};
const int dy[4] = {0,1,0,-1};

//for文
#define FOR(i,a,b) for(ll i = (ll)a;i < (ll)b;i++)
#define FORE(i,a,b) for(ll i = (ll) a;i <= (ll) b;i++ )
#define REP(i,size)  for(ll i =(ll)0;i<size;i++)
#define REPE(i,size)  for(ll i =(ll)0;i<=size;i++)
#define REPR(i,size)  for(ll i =(ll)size;i>=0;i--)
#define FOREACH(it, vec) for (auto it = vec.begin(); it != vec.end(); it++)

//ソート
#define ALL(vec) (vec).begin(),(vec).end()
#define SORT(vec) sort(ALL(vec))
#define SORTA(arr) sort(arr,arr + (sizeof(arr) / sizeof(ll)))
#define INVSORT(vec) sort((vec).rbegin(),(vec).rend())
#define REV(vec) reverse(ALL(vec))
#define REVA(arr) reverse(arr,arr + (sizeof(arr) / sizeof(ll)))
#define INVSORTA(arr) sort(arr,arr + (sizeof(arr) / sizeof(ll))),REVA(arr)

//最大値最小値
#define MAX(vec) *max_element(ALL(vec))
#define UNIQ(vec) SORT(vec);vec.erase(unique(ALL(vec)),vec.end())
#define MIN(vec) *min_element(ALL(vec))

//出力
#define printl(a) cout << a << "\n"
#define print(a) cout << a
#define OUT(a) printf("%lld\n",a)
#define OUTA(array) REP(i,sizeof(array)/sizeof(ll)) printf("%lld\n",array[i])
#define OUTV(vec) REP(i,vec.size()) printf("%lld\n",vec[i])
#define SP printf(" ")

//入力
#define IN(x) scanf("%lld",&x)
#define INV(vec) REP(i,vec.size()) scanf("%lld",&vec[i])
#define INA(array) REP(i,sizeof(array)/sizeof(ll)) scanf("%lld",array + i)
#define INS(x) cin >> x
#define INCH(x) scanf(" %c",&x)

//型
#define P pair
#define vp vector<P>
#define F first
#define S second

//その他
#define PB push_back
#define MP make_pair
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,-1,sizeof(a))
#define INFI(a) memset(a,INF,sizeof(a))
#define MEM(a,b) memset(a,(b),sizeof(a))

//関数
template<class T> inline void amax(T &a,const T &b) { if(a < b) a = b; }
template<class T> inline void amin(T &a, const T &b) { if(a > b) a = b; }
/*
struct structure{
    ll num1,num2,string s;
    bool operator<(const rest &another) const{
        return this->s < another.s;
    }
    structure(ll a,ll b,string s){
        this->num1 = a,this->num2=b,this->s=s;
    }
};a
*/
//特殊
//#define int ll
#define _CRT_SECURE_NO_WARNINGS

#define DEBUG

#ifdef DEBUG
#define debugl(x) cerr << #x << ":" << x<<"\n"
#define debug(x)  cerr << x
#define  debugV(V) REP(i,V.size()){cerr << i << ":" << V[i]<<endl;}
#define  debugA(A) REP(i,sizeof(A)/sizeof(ll)){cerr << i << ":" << V[i]<<endl;}
#else
#define debug(x)
#define debugV(x)
#define debugA(x)
#endif



signed main(){
    ll H,W;
    IN(H);IN(W);
    char a;
    vec2 dis(W,vec(H,-1));
    ll ans = 0;
    queue<P<int,int>> pos;

    FOR(i,0,H){
        FOR(j,0,W){
            INCH(a);
            if(a == '#') {
                pos.push(MP(j,i));
                dis[j][i] = 0;
            }
        }
    }

    pair<int,int> look;
    int look_x;
    int look_y;
    int next_x;
    int next_y;
    while(!pos.empty()) {
        look = pos.front();
        pos.pop();
        look_x = look.F;
        look_y = look.S;
        amax(ans,dis[look_x][look_y]);
        FOR(i,0,4){
            next_x = look_x + dx[i];
            next_y = look_y + dy[i];
            if(next_x < 0 or next_x >= W or next_y < 0 or next_y >= H ) continue;
            if(dis[next_x][next_y] == -1){
                dis[next_x][next_y] = dis[look_x][look_y] + 1;
                pos.push(MP(next_x,next_y));
            }
        }
    }


    OUT(ans);
}


Measured memory_bytes: 16272