#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "algo/debug.h"
#endif
using i64 = long long;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
static int H, W;
cin >> H >> W;
static vector<string> snow(H);
for (int i = 0; i < H; ++i) {
cin >> snow[i];
}
const int inf = (int) 1e9;
vector dist(H, vector<int>(W, inf));
struct cell {
int x, y;
bool valid() {
return x >= 0 && x < H && y >= 0 && y < W && snow[x][y] != '.';
};
};
deque<cell> dq;
auto ad = [&](cell c, int add, int extra = 0) {
if (extra) {
dq.push_back(c);
}
else {
dq.push_front(c);
}
dist[c.x][c.y] = add + extra;
};
ad(cell{0, 0}, 1);
vector<int> dx{1, -1, 0, 0};
vector<int> dy{0, 0, 1, -1};
int ans = 1;
while (!dq.empty()) {
auto c = dq.front();
dq.pop_front();
ans = max(ans, dist[c.x][c.y]);
for (int i = 0; i < 4; ++i) {
int x = c.x + dx[i], y = c.y + dy[i];
auto newc = cell{x, y};
if (!newc.valid() || dist[x][y] < inf) {
continue;
}
ad(newc, dist[c.x][c.y], (int) (snow[x][y] != snow[c.x][c.y]));
}
}
cout << ans << '\n';
}
/* stuff you should look for
* int overflow, array bounds
* special cases (n=1?)
* do smth instead of nothing and stay organized
* WRITE STUFF DOWN
* DON'T GET STUCK ON ONE APPROACH
*/
