#include<iostream>
using namespace std;

char mat[3000][3000];
int dp[3000][3000] = {};

int main(){
    int h, w;
    scanf("%d %d\n", &h, &w);

    for(int i = 0; i < h; i++){
        for(int j = 0; j < w; j++){
            scanf("%c ", &mat[i][j]);
        }
    }

    for(int i = 0; i < h; i++)  if(mat[i][0] == '#')    dp[i][0] = 1;
    for(int j = 0; j < w; j++)  if(mat[0][j] == '#')    dp[0][j] = 1;

    int ans = 1;
    for(int i = 1; i < h; i++){
        for(int j = 1; j < w; j++){
            if(mat[i][j] == '.')    continue;
            dp[i][j] = min(dp[i-1][j-1], min(dp[i-1][j], dp[i][j-1])) + 1;
            ans = max(ans, dp[i][j]);
        }
    }

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