#include <bits/stdc++.h>

using namespace std;

const int MAXN = 3005;

int H, W;
int f[MAXN][MAXN];
bool c[MAXN][MAXN];

int main()
{
    ios_base::sync_with_stdio(0); cin.tie(0);

    cin >> H >> W;
    char t;
    for (int i = 1; i <= H; ++i)
    for (int j = 1; j <= W; ++j) {
        cin >> t;
        if (t=='#') c[i][j] = 1;
    }
    int ans = 0;
    for (int i = 1; i <= H; ++i)
    for (int j = 1; j <= W; ++j) {
        if (c[i][j]) f[i][j] = min(f[i - 1][j - 1], min(f[i][j - 1], f[i - 1][j])) + 1;
        ans = max(ans, f[i][j]);
    }
    cout << (ans + 1) / 2;

    return 0;
}