// Ali
// #pragma GCC optimize("Ofast,unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define int long long int
#define ins insert
#define pii pair<int,int>
#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 = 1e6+23, inf = 2e9, LG = 19,pr = 31;
vector<int> graph[sze];
const int dx[4]={-1,0,1,0};
const int dy[4]={0,1,0,-1};
const int asz = 4e3 +5;
int cp[asz][asz];
int n,m;
int used[sze];
int check(int a,int b){
return (a>=0 && a<n && b>=0 && b<m);
}
void fast(){
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++){
cin>>arr[i][j];
}
}
int ct =1 ;
for(int i=0;i<n;i++){
for(int j=0;j<m;j++){
if(!cp[i][j] && arr[i][j]!='.'){
cp[i][j] = (ct++);
queue<pair<int,int>> q;
q.push({i,j});
while(!q.empty()){
pair<int,int> node = q.front();q.pop();
for(int x=0;x<4;x++){
if(check(node.first + dx[x], node.second + dy[x]) && !cp[node.first + dx[x]][node.second + dy[x]] && arr[node.first + dx[x]][node.second +dy[x] ]==arr[i][j]){
cp[node.first + dx[x]][node.second + dy[x]] = cp[i][j];
q.push({node.first + dx[x],node.second + dy[x]});
}
}
}
}
}
}
int a,b;
set<pair<int,int>> var;
for(int i=0;i<n;i++){
for(int j=0;j<m;j++){
if(cp[i][j]){
a = cp[i][j];
for(int x =0;x<4;x++){
if(check(i + dx[x], j + dy[x]) && cp[i + dx[x]][j+ dy[x]] && cp[i][j]!=cp[i + dx[x]][j + dy[x]]){
b = cp[i+dx[x]][j + dy[x]];
if(var.find({min(a,b),max(a,b)})==var.end()){
var.ins({min(a,b),max(a,b)});
graph[a].pb(b);
graph[b].pb(a);
}
}
}
}
}
}
int ans=1;
queue<pair<int,int>> q;
q.push({1,1});
used[1]=1;
while(!q.empty()){
pii node = q.front();q.pop();
ans=max(ans,node.second);
for(auto v:graph[node.first]){
if(!used[v]){
used[v]=1;
q.push({v,node.second +1});
}
}
}
putr(ans);
}
/*
5 8
FFR.....
.FRRR...
.FFFFF..
..RRRFFR
.....FFF
1 2
2 1 3
*/
signed main(){
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int tt = 1;
// cin>>tt;
while(tt--){
fast();
}
return 0;
}
