#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 = 4e3 +5, inf = INT_MAX, LL = 30;
int comp[sze][sze];
int used[sze][sze];
char arr[sze][sze];
const int dx[4] = {-1,1,0,0};
const int dy[4] = {0,0,-1,1};
int ct=0;
int n,m;
void dfs(int i,int j,char ch){
if(i<0 || i==n || j<0 || j==m || comp[i][j]!=-1 || arr[i][j]!=ch){
return;
}
comp[i][j]=ct;
for(int x =0;x<4;x++){
dfs(i+dx[x],j+dy[x],ch);
}
}
int a,b;
set<pair<int,int>> lst;
void dfs2(int i,int j){
if(i<0 || i==n || j<0 || j==m || comp[i][j]==-1 || used[i][j]){
return;
}
used[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]){
lst.ins({min(comp[a][b],comp[i][j]),max(comp[a][b],comp[i][j])});
}
dfs2(a,b);
}
}
void rush(){
cin>>n>>m;
for(int i=0;i<n;i++){
for(int j=0;j<m;j++){
comp[i][j]=-1;
cin>>arr[i][j];
}
}
int u,v;
for(int i=0;i<n;i++){
for(int j=0;j<m;j++){
if(arr[i][j]!='.' && comp[i][j]==-1){
dfs(i,j,arr[i][j]);
ct++;
}
}
}
vector<int> graph[ct+11];
for(int i=0;i<=ct;i++){
graph[i].clear();
}
dfs2(0,0);
for(auto v:lst){
graph[v.first].pb(v.second);
graph[v.second].pb(v.first);
}
int ans=0;
queue<pair<int,int>> q;
vector<int> used(ct,0);
q.push({0,1});
used[0]=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;
}
