#include <bits/stdc++.h>
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define FORR(i,a,b) for (int i=(a);i>=(b);i--)
#define pb push_back
#define pcnt __builtin_popcount
#define show(x) cout<<#x<<" = "<<x<<endl;
#define maxs(x,y) x = max(x,y)
#define mins(x,y) x = min(x,y)
#define fi first
#define se second
#define rng(a) a.begin(),a.end()
#define each(it,c) for(__typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define sz(x) (int)(x).size()
#define mp make_pair

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vpii;
typedef set<int> si;
typedef pair<ll,ll> pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<pll> vpll;
typedef set<ll> sl;
template<typename T>string join(vector<T>&v)
{stringstream s;FOR(i,0,sz(v))s<<' '<<v[i];return s.str().substr(1);}
ll gcd(ll a,ll b){if(a>b)swap(a,b);for(;a>0;b%=a,swap(a,b));return b;}
int modpow(ll a,ll n,int m){if(a==0)return a;ll p=1;for(;n>0;n/=2,a=a*a%m)if(n&1)p=p*a%m;return(int)p;}
void dout(double d){printf("%.12f\n",d);}

const int iinf = 1e9;
const ll linf = 1e18;
const int mod = 1e9+7;
const double pi = acos(-1);
const double eps = 1e-10;

int h, w, t[3002][3002];
string s;
main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> h >> w;
  h += 2;
  w += 2;
  FOR(i, 0, w){
    t[0][i] = i+1;
    t[h-1][i] = i+1;
  }
  FOR(i, 1, h-1){
    cin >> s;
    int su = 1;
    t[i][0] = su;
    FOR(j, 1, w-1){
      if(s[j-1] == '.') su++;
      t[i][j] = su;
    }
    t[i][w-1] = su+1;
  }
  FOR(j, 0, w){
    int su = 0;
    FOR(i, 0, h){
      su += t[i][j];
      t[i][j] = su;
    }
  }
  int ans = 0;
  FOR(i, 1, h-1)FOR(j, 1, w-1)if(t[i][j] + t[i-1][j-1] == t[i-1][j] + t[i][j-1]){
    int l = 0, r = min(h, w), mid, il, ir, jl, jr;
    while(r-l > 1){
      mid = (l+r)/2;
      il = i - mid;
      ir = i + mid;
      jl = j - mid;
      jr = j + mid;
      if(0 < il && ir < h-1 && 0 < jl && jr < w-1 && t[ir][jr] + t[il-1][jl-1] == t[ir][jl-1] + t[il-1][jr]){
        l = mid;
      }else{
        r = mid;
      }
    }
    maxs(ans, r);
  }
  cout << ans << endl;
  return 0;
}