#include <bits/stdc++.h>
using namespace std;
using ll = long long;
signed main() {
cin.tie(0)->sync_with_stdio(0);
int n, m;
cin >> n >> m;
vector<string> mat(n);
for (string& s : mat) {
cin >> s;
}
auto id = [m](int i, int j) { return i* m + j;};
vector<vector<pair<int,int>>> adj(n * m);
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
for (auto [di, dj] : {pair{0, -1}, {0, 1}, {-1, 0}, {1, 0}}) {
int ni = i + di;
int nj = j + dj;
if (clamp(ni, 0,n-1) == ni && clamp(nj, 0, m-1) == nj) {
if (mat[ni][nj] == '.' || mat[i][j] == '.') continue;
int c = mat[i][j] != mat[ni][nj];
adj[id(i, j)].push_back({id(ni, nj), c});
}
}
}
}
static constexpr int inf = 1e9;
vector<int> dist(n*m, inf), vis(n*m, 0);
int res = 0;
vector<int> edge;
auto dfs = [&](int i, int d) -> void {
vector<int> stk{i};
while (stk.size()) {
int i = stk.back();
stk.pop_back();
vis[i] = 1;
dist[i] = d;
for (auto [e, ec]: adj[i]) {
if (vis[e]) continue;
if (ec) {vis[e] = 1, edge.push_back(e);}
else
stk.push_back(e);
}
res = max(res, dist[i]);
}
};
dfs(0, 1);
for (int d = 2; edge.size(); d ++) {
auto cur = std::move(edge);
for (int k : cur)
dfs(k, d);
}
cout << res << '\n';
return 0;
}
