#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
const int MAX = 3030;
int dp[MAX][MAX];
signed main(){
  int h,w;
  cin>>h>>w;
  vector<string> s(h);
  for(int i=0;i<h;i++) cin>>s[i];

  memset(dp,0,sizeof(dp));
  for(int i=h-1;i>=0;i--)
    for(int j=w-1;j>=0;j--)
      if(s[i][j]=='#')
        dp[i][j]=min({dp[i+1][j],dp[i][j+1],dp[i+1][j+1]})+1;  
  
  int ans=0;
  for(int i=0;i<h;i++)
    for(int j=0;j<w;j++)
      chmax(ans,dp[i][j]);

  cout<<(ans+1)/2<<endl;
  return 0;
}