#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int label = 1, n, m;
const int mxN = 4e3+10;
const int mxN1 = 2e7+10;
vector<int> adj[mxN1];
bool vis[mxN][mxN];
char matrix[mxN][mxN];
int idx[mxN][mxN], d[mxN1];
vector<int> xy = {1, 0, -1, 0}, yx = {0, 1, 0, -1};
void dfs(int i, int j) {
char c = matrix[i][j];
idx[i][j] = label;
vis[i][j] = true;
for(int k = 0; k < 4; k++) {
int a = i + xy[k], b = j + yx[k];
if(a < 0 || a >= n || b < 0 || b >= m) continue;
if(matrix[a][b] == '.') continue;
if(matrix[a][b] == c) {
if(!vis[a][b]) dfs(a, b);
}
else {
if(vis[a][b]) {
adj[label].push_back(idx[a][b]);
adj[idx[a][b]].push_back(label);
}
}
}
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m;
for(int i = 0; i < n; i++) {
for(int j = 0; j < m; j++) {
cin >> matrix[i][j];
}
}
for(int i = 0; i < n; i++) {
for(int j = 0; j < m; j++) {
if(matrix[i][j] != '.' && !vis[i][j]) {
dfs(i, j);
++label;
}
}
}
queue<int> q;
q.push(1);
d[1] = 1;
int ans = 0;
while(!q.empty()) {
auto node = q.front();
q.pop();
ans = max(ans, d[node]);
for(auto it : adj[node]) {
if(d[it] == 0) {
d[it] = d[node]+1;
q.push(it);
}
}
}
cout << ans;
}
