#include <string>
#include <vector>
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<stack>
#include<queue>
#include<cmath>
#include<algorithm>
#include<functional>
#include<list>
#include<deque>
#include<bitset>
#include<set>
#include<map>
#include<unordered_map>
#include<cstring>
#include<sstream>
#include<complex>
#include<iomanip>
#include<numeric>
#include<cassert>
#define X first
#define Y second
#define pb push_back
#define rep(X,Y) for (int (X) = 0;(X) < (Y);++(X))
#define reps(X,S,Y) for (int (X) = S;(X) < (Y);++(X))
#define rrep(X,Y) for (int (X) = (Y)-1;(X) >=0;--(X))
#define repe(X,Y) for ((X) = 0;(X) < (Y);++(X))
#define peat(X,Y) for (;(X) < (Y);++(X))
#define all(X) (X).begin(),(X).end()
#define rall(X) (X).rbegin(),(X).rend()
#define eb emplace_back
#define UNIQUE(X) (X).erase(unique(all(X)),(X).end())

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
template<class T> using vv=vector<vector<T>>;
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"{"; rep(i,t.size()) {os<<t[i]<<",";} os<<"}"<<endl; return os;}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}
template<class T> inline bool MX(T &l,const T &r){return l<r?l=r,1:0;}
template<class T> inline bool MN(T &l,const T &r){return l>r?l=r,1:0;}
const ll MOD=1e9+7;

int dst[3123][3123];
int sum[3123][3123];
int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
bool full(int x,int y,int X,int Y){
  ++X; ++Y;
  if(x<0 || y<0 || X>3100 || Y>3100) return 0;
  return sum[Y][X]+sum[y][x]-sum[y][X]-sum[Y][x]==(X-x)*(Y-y);
}
int main(){
  ios_base::sync_with_stdio(false);
  cout<<fixed<<setprecision(0);
  int y,x;
  cin>>y>>x;
  vector<string> mp(y);
  rep(i,y) cin>>mp[i];
  rep(i,y)rep(j,x) if(mp[i][j]=='#')++sum[i+1][j+1];
  rep(i,y+1)rep(j,x+1) if(i) sum[i][j]+=sum[i-1][j];
  rep(i,y+1)rep(j,x+1) if(j) sum[i][j]+=sum[i][j-1];
  int re=0;
  //cout<<full(0,2,4,6)<<endl;
  rep(i,y)rep(j,x)if(mp[i][j]=='#'){
    int l=0,r=1500,m;
    while(r-l>1){
      m=(l+r)/2;
      if(full(j-m,i-m,j+m,i+m)) l=m;
      else r=m;
    }
    MX(re,r);
    //if(!j) cout<<endl;
    //cout<<r<<",";
  }
  cout<<re<<endl;
  return 0;
}