#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
inline int in() { int x; scanf("%d",&x); return x;}
template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,sz(v))i>>v[j];return i;}
template<typename T>string join(const v(T)&v)
{stringstream s;rep(i,sz(v))s<<' '<<v[i];return s.str().substr(1);}
template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
{if(sz(v))o<<join(v);return o;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
{return o<<v.fi<<","<<v.se;}
template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame { puts("-1"); return 0;}
#define yn {puts("Yes");}else{puts("No");}
const int MX = 200005;
#define RET(ans) {cout<<ans<<endl;return 0;} 

    // 二次元ベクターの基本
    /*
    vector<vector<int>> dp; // 宣言
    dp.resize(n); // 1次元めの要素数決定
    dp[i].push_back(int); // プッシュバック
    rep(i,n){
        sort(dp[i].begin(),dp[i].end()); // 二次元めを昇順ソート
    }
    */

    // 整数スキャン（複数）
    /*
    int x,y,z;
    scanf("%d%d%d",&x,&y,&z);

    // n個の整数のスキャン
    /*
    ll a[n] = {};
    rep(i,n){
        scanf("%lld",&a[i]);
    }
    */

    // 文字列スキャン
    /*
    string s; cin >> s;
    */

    // n個の文字列スキャン
    /*
    vector<string> slist;
    rep(i,n){
        string s; cin >> s;
        slist.push_back(s);
    }
    */

int main() {
    int h,w;
    cin >> h >> w;
    string s[h];
    rep(i,h){
        cin >> s[i];
    }

    int a[h][w];
    int b[h][w];

    rep(i,h){
        rep(j,w){
            if(s[i][j]=='#'){
                a[i][j] = 0;
                b[i][j] = 0;
            }else{
                a[i][j] = 9999;
            b[i][j] = 9999;
            }
            
        }
    }

    // i==0
    int x = 9999;
    int flag = 0;
    rep(j,w){
        if(s[0][j]=='#'){
            flag = 1;
            a[0][j] = 0;
            x = j;
        }else{
            if(flag==1){
                a[0][j] = j-x;
            }
        }
    }
    flag = 0;
    drep(j,w){
        if(s[0][j]=='#'){
            flag = 1;
            a[0][j] = 0;
            x = j;
        }else{
            if(flag==1){
                a[0][j] = min(a[0][j],abs(j-x));
            }
        }
    }

    srep(i,1,h){
        flag = 0;
        x = 9999;
        rep(j,w){
            if(s[i][j]=='#'){
                flag = 1;
                a[i][j] = 0;
                x = j;
            }else{
                if(j==0){
                    a[i][j] = a[i-1][j] + 1;
                }else{
                    a[i][j] = min(a[i-1][j]+1,a[i][j-1]+1);
                }
            }
        }
        drep(j,w){
           if(s[i][j]=='#'){
                flag = 1;
                a[i][j] = 0;
                x = j;
            }else{
                if(j==w-1){
                    a[i][j] = min(a[i-1][j] + 1,a[i][j]);
                }else{
                    a[i][j] = min(a[i][j],a[i][j+1]+1);
                }
           } 
        }
    }


    srep(j,1,w){
        b[h-1][j] = min(b[h-1][j],b[h-1][j-1]+1);
    }
    drep(j,w){
        if(j!=w-1){
            b[h-1][j] = min(b[h-1][j],b[h-1][j+1]+1);
        }
    }

    for(int i = h-2;i>=0;i--){
        rep(j,w){
            if(s[i][j]=='#'){
                flag = 1;
                b[i][j] = 0;
                x = j;
            }else{
                if(j==0){
                    b[i][j] = b[i+1][j] + 1;
                }else{
                    b[i][j] = min(b[i+1][j]+1,b[i][j-1]+1);
                }
            }
        }
        drep(j,w){
           if(s[i][j]=='#'){
                flag = 1;
                b[i][j] = 0;
                x = j;
            }else{
                if(j==w-1){
                    b[i][j] = min(b[i+1][j] + 1,b[i][j]);
                }else{
                    b[i][j] = min(b[i][j],b[i][j+1]+1);
                }
           } 
        }
    }
    



    int ans = 0;

    rep(i,h){
        rep(j,w){
            int temp = min(a[i][j],b[i][j]);
            ans = max(ans,temp);
        }
    }
    
    cout << ans << endl;
    return 0;
}
 
 


Measured memory_bytes: 9088