#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <stack>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;

int H, W;
string field[3005];
int dp[3005][3005];

int main(){
  ios::sync_with_stdio(false);

  cin >> H >> W;
  rep(i,H){
    cin >> field[i];
  }
  rep(i,H){
    if(field[i][0]=='#') dp[i][0] = 1;
  }
  rep(j,W){
    if(field[0][j]=='#') dp[0][j] = 1;
  }

  REP(i,1,H){
    REP(j,1,W){
      if(field[i][j]=='.'){
        dp[i][j] = 0;
      }else{
        int sz = dp[i-1][j-1] + 1;
        if(dp[i][j-1]+1<sz){
          sz = dp[i][j-1]+1;
        }
        if(dp[i-1][j]+1<sz){
          sz = dp[i-1][j]+1;
        }
        dp[i][j] = sz;
      }
    }
  }

  int ret = 0;
  rep(i,H){
    rep(j,W){
      ret = max(ret, dp[i][j]);
    }
  }
  cout << (ret+1)/2 << endl;
  
  return 0;
}