#include <bits/stdc++.h>
using namespace std;
#define int long long int
#define ins insert
#define pb push_back
#define endl '\n'
#define putr(x) cout<<x<<endl;return;
#define all(x) x.begin(),x.end()
const int mod = 1e9 +7, sze = 1e5 +5, inf = INT_MAX, LL = 30;
int comp[4005][4005];
const int dx[4] = {-1,1,0,0};
const int dy[4] = {0,0,-1,1};
void rush(){
int n,m;
cin>>n>>m;
vector<vector<char>> arr(n,vector<char>(m));
for(int i=0;i<n;i++){
for(int j=0;j<m;j++){
comp[i][j]=0;
cin>>arr[i][j];
}
}
int x,y;
int ct=1,a,b,u,v;
for(int i=0;i<n;i++){
for(int j=0;j<m;j++){
if(arr[i][j]!='.'){
if(!comp[i][j]){
comp[i][j]= (ct++);
}
for(int k=0;k<4;k++){
x= i + dx[k];
y = j + dy[k];
if( x>=0 && y>=0 && x<n && y<m){
if(arr[i][j] == arr[x][y]){
comp[x][y]=comp[i][j];
}
}
}
}
}
}
vector<int> graph[ct+10];
for( int i=0;i<=ct;i++){
graph[i]=vector<int>();
}
for(int i =0;i<n;i++){
for(int j=0;j<m;j++){
if(comp[i][j]!=-1){
for(int x= 0; x<4;x++){
a = i + dx[x];
b = j + dy[x];
if(a>=0 && a<n && b>=0 && b<m && comp[a][b] !=-1 && comp[a][b]!=comp[i][j]){
u = min(comp[i][j],comp[a][b]);
v =max(comp[i][j],comp[a][b]);
// cout<<u<<" "<<v<<endl;
graph[u].pb(v);
graph[v].pb(u);
}
}
}
}
}
int ans=0;
queue<pair<int,int>> q;
vector<int> used(ct,0);
q.push({1,1});
used[1]=1;
while(!q.empty()){
auto node = q.front();
ans=max(ans,node.second);
q.pop();
for(auto v:graph[node.first]){
if(!used[v]){
used[v]=1;
q.push({v,node.second+1});
}
}
}
putr(ans);
}
signed main(){
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int tt = 1;
// cin>>tt;
while(tt--){
rush();
}
return 0;
}
