#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
namespace my{
#define eb emplace_back
#define LL(...) ll __VA_ARGS__;lin(__VA_ARGS__)
#define FO(n) for(ll ij=n;ij--;)
#define FOR(i,...) for(auto[i,i##stop,i##step]=range(0,__VA_ARGS__);i<i##stop;i+=i##step)
#define fo(i,...) FO##__VA_OPT__(R)(i __VA_OPT__(,__VA_ARGS__))
#define fe(a,i,...) for(auto&&__VA_OPT__([)i __VA_OPT__(,__VA_ARGS__]):a)
#define single_testcase void solve();}int main(){my::io();my::solve();}namespace my{
void io(){cin.tie(nullptr)->sync_with_stdio(0);cout<<fixed<<setprecision(15);}
using ll=long long;
constexpr auto range(bool s,auto...a){array<ll,3>r{0,0,1};ll I=0;((r[I++]=a),...);if(!s&&I==1)swap(r[0],r[1]);r[0]-=s;return r;}
constexpr char newline=10;
constexpr char space=32;
constexpr auto zz(auto x){return x<0?-x:x;}
bool amin(auto&a,const auto&b){return a>b?a=b,1:0;}
auto min(const auto&...a){return min(initializer_list<common_type_t<decltype(a)...>>{a...});}

template<class A,class B>struct pair{
  A a;B b;
  pair()=default;
  pair(A a,B b):a(a),b(b){}
  pair(const std::pair<A,B>&p):a(p.first),b(p.second){}
  auto operator<=>(const pair&)const=default;
  pair operator+(const pair&p)const{return{a+p.a,b+p.b};}
  friend ostream&operator<<(ostream&o,const pair&p){return o<<p.a<<space<<p.b;}
};

template<class T,class U>ostream&operator<<(ostream&o,const std::pair<T,U>&p){return o<<p.first<<space<<p.second;}

template<class V>concept vectorial=is_base_of_v<vector<typename V::value_type>,V>;
template<class T>struct vec_attr{using core_type=T;static constexpr int d=0;};
template<vectorial V>struct vec_attr<V>{using core_type=typename vec_attr<typename V::value_type>::core_type;static constexpr int d=vec_attr<typename V::value_type>::d+1;};
template<class T>using core_t=vec_attr<T>::core_type;
template<class V>istream&operator>>(istream&i,vector<V>&v){fe(v,e)i>>e;return i;}
template<class V>ostream&operator<<(ostream&o,const vector<V>&v){fe(v,e)o<<e<<string(&e!=&v.back(),vectorial<V>?newline:space);return o;}

template<class V>struct vec:vector<V>{
  using vector<V>::vector;
  vec(const vector<V>&v){vector<V>::operator=(v);}

  vec&operator^=(const vec&u){this->insert(this->end(),u.begin(),u.end());return*this;}
  vec operator^(const vec&u)const{return vec{*this}^=u;}
  vec&operator+=(const vec&u){vec&v=*this;fo(i,v.size())v[i]+=u[i];return v;}
  vec&operator-=(const vec&u){vec&v=*this;fo(i,v.size())v[i]-=u[i];return v;}
  vec operator+(const vec&u)const{return vec{*this}+=u;}
  vec operator-(const vec&u)const{return vec{*this}-=u;}
  vec&operator++(){fe(*this,e)++e;return*this;}
  vec&operator--(){fe(*this,e)--e;return*this;}
  vec operator-()const{vec v=*this;fe(v,e)e=-e;return v;}

  auto scan(const auto&f)const{pair<core_t<V>,bool>r{};fe(*this,e)if constexpr(!vectorial<V>)r.b?f(r.a,e),r:r={e,1};else if(auto s=e.scan(f);s.b)r.b?f(r.a,s.a),r:r=s;return r;}
  auto min()const{return scan([](auto&a,const auto&b){a>b?a=b:0;;}).a;}
  vec zeta()const{vec v=*this;if constexpr(vectorial<V>)fe(v,e)e=e.zeta();fo(i,v.size()-1)v[i+1]+=v[i];return v;}

  auto rectangle_sum(ll a,ll b,ll c,ll d)const{
    static_assert(is_same_v<V,vec<core_t<V>>>);
    assert(0<=a&&a<=b&&b<=(ll)this->size());
    assert(0<=c&&c<=d&&d<=(ll)(*this)[0].size());
    core_t<V>r{};
    if(b&&d)r+=(*this)[b-1][d-1];
    if(a&&d)r-=(*this)[a-1][d-1];
    if(b&&c)r-=(*this)[b-1][c-1];
    if(a&&c)r+=(*this)[a-1][c-1];
    return r;
  }

  auto&rectangle_imos(ll a,ll b,ll c,ll d,core_t<V>x){
    (*this)[a][c]+=x;
    if(b<(ll)this->size())(*this)[b][c]-=x;
    if(d<(ll)(*this)[0].size())(*this)[a][d]-=x;
    if(b<(ll)this->size()&&d<(ll)(*this)[0].size())(*this)[b][d]+=x;
    return*this;
  }
};
template<class T=ll,size_t n,size_t i=0>auto make_vec(const ll(&s)[n],T x={}){if constexpr(n==i+1)return vec<T>(s[i],x);else{auto X=make_vec<T,n,i+1>(s,x);return vec<decltype(X)>(s[i],X);}}

void lin(auto&...a){(cin>>...>>a);}
auto sinen(const string&b="a"){string s;lin(s);vec<ll>r;fe(s,e)r.eb(b.size()==1?e-b[0]:b.find_first_of(e));return r;}
auto sinen(ll n,const string&b="a"){vec<vec<ll>>r;fo(n)r.eb(sinen(b));return r;}
template<char c=space>void pp(const auto&...a){ll n=sizeof...(a);((cout<<a<<string(--n>0,c)),...);cout<<newline;}

ll bsl(const auto&f,ll o,ll x){while(zz(o-x)>1)(f((o+x)/2)?o:x)=(o+x)/2;return o;}

namespace sgt{
template<class T>T min(T a,T b){return a<b?a:b;}
}

single_testcase
void solve(){
  LL(H,W);
  auto a=sinen(H,".#");
  auto sa=a.zeta();

  pp(bsl([&](ll K){
    auto b=make_vec({H,W});
    fo(i,H-K+1)fo(j,W-K+1)if(sa.rectangle_sum(i,i+K,j,j+K)==K*K)b.rectangle_imos(i,i+K,j,j+K,1);
    b=b.zeta();
    fo(i,H)fo(j,W)amin(b[i][j],1);
    return a==b;
  },1,min(H,W)+1));
}}