#include <bits/stdc++.h>
using namespace std;
//------------------------------DEFINE------------------------------
//******************************************************************
#define IOS ios_base::sync_with_stdio(false); cin.tie(0),cout.tie(0)
#define ll long long
#define pb push_back
#define F first
#define S second
#define INF 1e18
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define pii pair<int,int>
#define pll pair<ll,ll>
#define OK cout<<"Ok"<<endl;
#define MOD (ll)(1e9+7)
#define endl "\n"
//******************************************************************
//----------------------------FUNCTION------------------------------
//******************************************************************
ll gcd(ll a,ll b){
if(a>b) swap(a,b);
if(a==0) return a+b;
return gcd(b%a,a);
}
ll lcm(ll a,ll b){
return a/gcd(a,b)*b;
}
bool is_prime(ll n){
ll k=sqrt(n);
if(n==2) return true;
if(n<2||n%2==0||k*k==n) return false;
for(int i=3;i<=k;i+=2){
if(n%i==0){
return false;
}
}
return true;
}
//*****************************************************************
//--------------------------MAIN-CODE------------------------------
const int mxn=4000+5;
const int N=1e7;
int t=1,n,m,k,a,b,x[mxn][mxn],dis[N],color[mxn][mxn],colorr[N];
char arr[mxn][mxn];
char c[3];
map<pii,bool>edge;
vector<int>vt[N];
void dfs(int i,int j){
char cc=arr[i][j];
color[i][j]=1;
x[i][j]=k;
if(arr[i+1][j]==cc&&!color[i+1][j]){
dfs(i+1,j);
}
if(arr[i-1][j]==cc&&!color[i-1][j]){
dfs(i-1,j);
}
if(arr[i][j+1]==cc&&!color[i][j+1]){
dfs(i,j+1);
}
if(arr[i][j-1]==cc&&!color[i][j-1]){
dfs(i,j-1);
}
}
void solve(){
memset(arr,'.',sizeof(arr));
cin>>n>>m;
for(int i=1;i<=n;i++){
for(int j=1;j<=m;j++){
cin>>arr[i][j];
}
}
for(int i=1;i<=n;i++){
for(int j=1;j<=m;j++){
if(arr[i][j]!='.'&&!color[i][j]){
k++;
dfs(i,j);
}
}
}
for(int i=1;i<=n;i++){
for(int j=1;j<=m;j++){
if(arr[i][j]!=arr[i][j+1]&&arr[i][j]!='.'&&arr[i][j+1]!='.'){
if(!edge[{x[i][j],x[i][j+1]}]){
edge[{x[i][j],x[i][j+1]}]=1;
edge[{x[i][j+1],x[i][j]}]=1;
vt[x[i][j]].pb(x[i][j+1]);
vt[x[i][j+1]].pb(x[i][j]);
}
}
if(arr[i][j]!=arr[i][j-1]&&arr[i][j]!='.'&&arr[i][j-1]!='.'){
if(!edge[{x[i][j],x[i][j-1]}]){
edge[{x[i][j],x[i][j-1]}]=1;
edge[{x[i][j-1],x[i][j]}]=1;
vt[x[i][j]].pb(x[i][j-1]);
vt[x[i][j-1]].pb(x[i][j]);
}
}
if(arr[i][j]!=arr[i+1][j]&&arr[i][j]!='.'&&arr[i+1][j]!='.'){
if(!edge[{x[i][j],x[i+1][j]}]){
edge[{x[i][j],x[i+1][j]}]=1;
edge[{x[i+1][j],x[i][j]}]=1;
vt[x[i][j]].pb(x[i+1][j]);
vt[x[i+1][j]].pb(x[i][j]);
}
}
if(arr[i][j]!=arr[i-1][j]&&arr[i][j]!='.'&&arr[i-1][j]!='.'){
if(!edge[{x[i][j],x[i-1][j]}]){
edge[{x[i][j],x[i-1][j]}]=1;
edge[{x[i-1][j],x[i][j]}]=1;
vt[x[i][j]].pb(x[i-1][j]);
vt[x[i-1][j]].pb(x[i][j]);
}
}
}
}
queue<int>q;
q.push(1);
colorr[1]=1;
dis[1]=1;
int ans=1;
while(q.size()){
int u=q.front();
q.pop();
for(int v:vt[u]){
if(!colorr[v]){
dis[v]=dis[u]+1;
colorr[v]=1;
q.push(v);
ans=max(ans,dis[v]);
}
}
}
cout<<ans<<endl;
}
int main(){
IOS;
//cin>>t;
while(t--){
solve();
}
return 0;
}
