#include <bits/stdc++.h>
int main() {
std::ios_base::sync_with_stdio(false);
std::cin.tie(nullptr);
int n, m;
std::cin >> n >> m;
std::vector<std::string> g(n);
for (int i = 0; i < n; ++i) {
std::cin >> g[i];
}
std::vector<std::vector<int>> comp(n, std::vector<int>(m, 0));
int num = 0;
std::function<void(int, int, char)> dfs=[&](int x, int y, char c) {
if (x < 0 || x >= n || y < 0 || y >= m || comp[x][y] || g[x][y] != c)
return;
comp[x][y] = num;
dfs(x + 1, y, c);
dfs(x - 1, y, c);
dfs(x, y + 1, c);
dfs(x, y - 1, c);
};
g.clear();
for (int x = 0; x < n; ++x) {
for (int y = 0; y < m; ++y) {
if (comp[x][y] || g[x][y] == '.')
continue;
num++;
dfs(x, y, g[x][y]);
}
}
std::vector<std::vector<int>> adj(num + 1);
auto addcomp=[&](int i, int j) {
auto add=[&](int x, int y) {
if (x < 0 || x >= n || y < 0 || y >= m)
return;
if (comp[x][y] && comp[x][y] != comp[i][j]) {
adj[comp[x][y]].push_back(comp[i][j]);
adj[comp[i][j]].push_back(comp[x][y]);
}
};
add(i - 1, j);
add(i + 1, j);
add(i, j + 1);
add(i, j - 1);
};
for (int i = 0; i < n; ++i) {
for (int j = 0; j < m; ++j) {
if (comp[i][j])
addcomp(i, j);
}
}
comp.clear();
std::vector<int> d(num + 1, 0);
std::queue<int> q;
q.push(1);
d[1] = 1;
int ans = 1;
while (!q.empty()) {
int u = q.front();
q.pop();
for (int v : adj[u]) {
if (d[v] == 0) {
q.push(v);
d[v] = d[u] + 1;
ans = std::max(ans, d[v]);
}
}
}
std::cout << ans << "\n";
return 0;
}
