#include <bits/stdc++.h>
using namespace std;
set<int> adj[16000000];
vector<int> topo;
int vis[16000000];
void dfs(int x){
if(vis[x])return;
vis[x]=1;
for(set<int>::iterator it=adj[x].begin();it!=adj[x].end();++it){
if(vis[(*it)])continue;
dfs(*it);
}
topo.push_back(x);
}
int main(){
int h,w;
scanf("%d%d",&h,&w);
char grid[h+5][w+5];
for(int i=0;i<h;i++){
scanf("%s",grid[i]);
}
queue<pair<int,int> > q;
int com[h][w];
memset(com,-1,sizeof(com));
int c=0;
int dx[]={0,0,1,-1},dy[]={1,-1,0,0};
for(int i=0;i<h;i++){
for(int j=0;j<w;j++){
if(com[i][j]==-1){
q.push(make_pair(i,j));
com[i][j]=c;
while(!q.empty()){
pair<int,int> cur=q.front();q.pop();
for(int k=0;k<4;k++){
int nx=cur.first+dx[k],ny=cur.second+dy[k];
if(nx<0||ny<0||nx>=h||ny>=w)continue;
if(grid[nx][ny]==grid[cur.first][cur.second]&&com[nx][ny]==-1){
com[nx][ny]=c;
q.push(make_pair(nx,ny));
}
}
}
c++;
}
}
}
q.push(make_pair(0,0));
while(!q.empty()){
pair<int,int> cur=q.front();q.pop();
///printf("%d %d\n",cur.first,cur.second);
if(grid[cur.first][cur.second]=='~')continue;
for(int i=0;i<4;i++){
int nx=cur.first+dx[i],ny=cur.second+dy[i];
if(0<=nx&&nx<h&&0<=ny&&ny<w){
if(grid[nx][ny]=='~')continue;
if(grid[nx][ny]=='.'){
adj[com[cur.first][cur.second]].insert(com[nx][ny]);
continue;
}
if(grid[nx][ny]==grid[cur.first][cur.second]){
q.push(make_pair(nx,ny));
continue;
}
///printf("%d %d %d %d\n",cur.first,cur.second,nx,ny);
adj[com[cur.first][cur.second]].insert(com[nx][ny]);
q.push(make_pair(nx,ny));
}
}
grid[cur.first][cur.second]='~';
}
dfs(0);
reverse(topo.begin(),topo.end());
int dist[c]; memset(dist,-1,sizeof(dist));
dist[0]=0;
for(int i=0;i<c;i++){
int x=topo[i];
for(set<int>::iterator it=adj[x].begin();it!=adj[x].end();++it){
dist[(*it)]=max(dist[(*it)],dist[x]+1);
}
}
int ans=0;
for(int i=0;i<h;i++){
for(int j=0;j<w;j++){
if(grid[i][j]=='.')ans=max(ans,dist[com[i][j]]);
}
}
printf("%d",ans);
}
