#include <bits/stdc++.h>  // include all standard C++ libraries

using namespace std;

// Loops
#define _overload3(_1,_2,_3,name,...) name
#define _rep(i,n) repi(i,0,n)
#define repi(i,a,b) for(int i=int(a);i<int(b);++i)
#define _rrep(i,n) rrepi(i,n,0)
#define rrepi(i,a,b) for(int i=int(a)-1;i>=int(b);--i)

#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#define rrep(...) _overload3(__VA_ARGS__,rrepi,_rrep,)(__VA_ARGS__)
#define each(xi, x) for (auto&& xi : x)
// Note: we can use rep(i,N) or rep(i,from,to)


// typedef
using ll = long long;
template <class T> using vec = vector<T>;
using vi   = vector<int>;
using vvi  = vector<vi>;
using vvvi = vector<vvi>;
using pii  = pair<int, int>;


// Constants

// Shorter repr for frequently used terms
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define Fi first
#define Se second


// Algorithms
#define all(x) (x).begin(), (x).end()
#define uniq(v) v.erase( unique(all(v)), v.end() )
#define perm(c) sort(all(c));for(bool c##p=1;c##p;c##p=next_permutation(all(c)))

template<class T> pair<T, size_t> max(vector<T> &x){ auto it = max_element(all(x)); return mp(*it, it-x.begin()); }
template<class T> pair<T, size_t> min(vector<T> &x){ auto it = min_element(all(x)); return mp(*it, it-x.begin()); }

template<class T> inline bool chmax(T &maxval, const T &newval) { if (maxval<newval) { maxval=newval; return 1; } return 0; }
template<class T> inline bool chmin(T &minval, const T &newval) { if (minval<newval) { minval=newval; return 1; } return 0; }

// Utilities
// Grid world utilities
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
#define inside(H,W,y,x) 0<=(x) && (x)<(W) && 0<=(y) && (y)<(H)

inline int in() {int x; cin >> x; return x;} // read int from cin
inline  ll IN() {ll x; cin >> x; return x;}  // read ll from cin



// Debug
#ifdef LOCAL
  #include "dump.hpp"
  #define debug(x) cerr << #x << ": " << x << '\n'
#else
  #define dump(...)
  #define debug(x)
#endif

// Paste snippets here!!
bool kakomi(int i, int j, vector<string> &A){
  if(0<=i-1 && A[i-1][j]=='.') return false;
  if(i+1<(int)A.size() && A[i+1][j]=='.') return false;
  if(0<=j-1 && A[i][j-1]=='.') return false;
  if(j+1<(int)A[i].size() && A[i][j+1]=='.') return false;
  return true;
}
// 

int main(){
  cin.tie(0); ios::sync_with_stdio(false);    // Magic for faster cin
  int H, W;
  cin >> H >> W;
  vector<string> A(H);
  stack<pii> s, t;
  int initwhite = 0;
  rep(i,H){
    cin >> A[i];
    for(int j=0;j<W;++j){
      if(A[i][j]=='#'){
        //if(!kakomi(i,j,A)) s.push(mp(i,j));
        s.push(mp(i,j));
      } else {
        initwhite++;
      }
    }
  }
  int ret = 0;
  stack<pii> *S, *T;
  while(initwhite){
    if(ret%2==0){
      S = &s; T = &t;
    } else {
      S = &t; T = &s;
    }
    while(!S->empty()){
      auto ij = S->top(); S->pop();
      int i = ij.first; int j = ij.second;
      for(int k=0;k<4;++k){
        int i0 = i+dy[k];
        int j0 = j+dx[k];
        if(inside(H,W,i0,j0) && A[i0][j0]=='.'){
          A[i0][j0] = '#';
          initwhite--;
          //if(!kakomi(i0,j0,A)) T->push(mp(i0,j0));
          T->push(mp(i0,j0));
        }
      }
    }
    ret++;
  }
  cout << ret << endl;
  return 0;
}


Measured memory_bytes: 9488