#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 = 4005, inf = INT_MAX, LL = 30;
const int dx[4] = {-1,1,0,0};
const int dy[4] = {0,0,-1,1};
int comp[sze][sze];
map<pair<int,int>,int> var;
vector<int> graph[sze*sze];
int used[sze*sze];
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 c=1,a,b;
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]= (c++);
}
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];
}
else{
if(arr[x][y]!='.' && comp[x][y]){
a = comp[x][y];
b = comp[i][j];
if(!var[make_pair(min(a,b),max(a,b))]){
var[make_pair(min(a,b),max(a,b))]=1;
graph[a].pb(b);
graph[b].pb(a);
}
}
}
}
}
}
}
}
int ans=1;
queue< pair<int,int>> q;
q.push({1,1});
while(!q.empty()){
pair<int,int> node =q.front();
q.pop();
if(!used[node.first]){
ans=max(ans,node.second);
used[node.first]=1;
for(auto v:graph[node.first]){
if(!used[v]){
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;
}
