// EMPTY
//
#include <stdio.h>
#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <map>
#include <array>
#include <deque>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <algorithm>
#include <stdlib.h>
#include <math.h>
#include <list>
#include <float.h>
#include <climits>
using namespace std;
void quit() {
cout.flush();
exit(0);
}
char grid[4002][4002];
map<int, unordered_set<int>> adj;
int id[4002][4002];
int curr_id = 1;
deque <pair<int, int>> dq;
vector<pair<int, int>> gto = {{1,0},{0,1},{-1,0},{0,-1}};
void flood(int a, int b){
id[a][b]=curr_id;
char c = grid[a][b];
grid[a][b]='.';
dq.clear();
dq.emplace_back(a, b);
while(!dq.empty()){
auto curr = dq.front();
dq.pop_front();
for(auto to : gto){
pair<int, int> nxt = {curr.first+to.first, curr.second+to.second};
if(grid[nxt.first][nxt.second] == c){
id[nxt.first][nxt.second]=curr_id;
grid[nxt.first][nxt.second] = '.';
dq.push_back(nxt);
}
if(id[nxt.first][nxt.second]!=0 && id[nxt.first][nxt.second] != curr_id){
adj[id[nxt.first][nxt.second]].insert(curr_id);
adj[curr_id].insert(id[nxt.first][nxt.second]);
}
}
}
curr_id++;
}
int main(void){
//freopen("qwer.in", "r", stdin);
//freopen("qwer.out", "w", stdout);
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n, m; cin >> n >> m;
for(int i = 0; i < 4002; i++){
for(int j = 0; j < 4002; j++){
grid[i][j]='.';
}
}
for(int i = 1; i <= n; i++){
string s; cin >> s;
for(int j = 1; j <= m; j++){
grid[i][j]=s[j-1];
}
}
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
if(grid[i][j] != '.'){
flood(i, j);
}
}
}
deque <int> dq;
dq.push_back(1);
vector <int> dist(curr_id, 1e9);
dist[1]=0;
while(!dq.empty()){
int curr = dq.front();
dq.pop_front();
for(int nxt : adj[curr]){
if(dist[curr]+1 < dist[nxt]){
dist[nxt]=dist[curr]+1;
dq.push_back(nxt);
}
}
}
int ans = 0;
for(int i = 1; i < curr_id; i++){
ans = max(ans, dist[i]);
}
cout << ans+1 << "\n";
quit();
}
