#include<bits/stdc++.h>

using namespace std;
typedef unsigned int uint;
typedef long long int ll;
typedef unsigned long long int ull;

#define debugv(v) printf("L%d %s => ",__LINE__,#v);for(auto e:v){cout<<e<<" ";}cout<<endl;
#define debugm(m) printf("L%d %s is..\n",__LINE__,#m);for(auto v:m){for(auto e:v){cout<<e<<" ";}cout<<endl;}
#define debuga(m,w) printf("L%d %s is => ",__LINE__,#m);for(int x=0;x<(w);x++){cout<<(m)[x]<<" ";}cout<<endl;
#define debugaa(m,w,h) printf("L%d %s is..\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){cout<<(m)[x][y]<<" ";}cout<<endl;}
#define ALL(v) (v).begin(),(v).end()
#define BIGINT 0x7FFFFFFF
#define E107 1000000007

template<typename T1,typename T2>
ostream& operator <<(ostream &o,const pair<T1,T2> p){o<<"("<<p.first<<":"<<p.second<<")";return o;}

int h,w;
char maze[5000][5000];
int dp[5005][5005];

int main(){
    int i,j,k,l;
    
    int maxi=0;
    cin>>h>>w;cin.ignore();
    
    for (i=1;i<=h;i++){
        gets(maze[i]);
        for (j=0;j<w;j++){
            if (maze[i][j]=='#'){
                maxi=max(dp[i][j]=min(dp[i-1][j-1],min(dp[i-1][j],dp[i][j-1]))+1,maxi);
            }
        }
    }
    
    cout<<(maxi+1)/2<<endl;


    return 0;
}