#include <bits/stdc++.h>
using namespace std;
int n, m;
char a[4005][4005];
int belongto[4005][4005];
int component[160000005];
bool visited[4005][4005];
set<int>neighbors[4005];
bool gothere[160000005];
int source=0;
char core;
const int dx[4]={0, 0, 1, -1};
const int dy[4]={1, -1, 0, 0};
int temp=0;
deque<int>dq;
int ans=0;
int d[160000005];
bool check(int i, int j){
if (1<=i && i<=n && 1<=j && j<=m) return true;
else return false;
}
void floodfill(int i, int j, char cur){
if (check(i, j)==false || visited[i][j] || a[i][j]!=cur || a[i][j]=='.') return;
visited[i][j]=true;
belongto[i][j]=temp;
for (int t=0; t<4; t++){
int u=i+dx[t];
int v=j+dy[t];
if (check(u, v) && belongto[u][v]!=0 && belongto[u][v]!=temp){
neighbors[temp].insert(belongto[u][v]);
neighbors[belongto[u][v]].insert(temp);
}
floodfill(u, v, cur);
}
}
int main(){
cin>>n>>m;
for (int i=1;i <=n; i++){
for (int j=1; j<=m; j++){
cin>>a[i][j];
if (i==1 && j==1){
core=a[i][j];
}
}
}
for (int i=1;i <=n; i++){
for (int j=1; j<=m; j++){
if (visited[i][j]==false && a[i][j]!='.'){
temp++;
d[temp]=1e9;
component[temp]=(a[i][j]!=core);
floodfill(i, j, a[i][j]);
}
}
}
/* for (int i=1; i<=temp; i++){
cout<<i<<" "<<"component: "<<component[i]<<" neighbors: ";
for (auto c:neighbors[i]){
cout<<c<<" ";
}
cout<<endl;
} */
d[1]=1;
dq.push_back(1);
while (!dq.empty()){
int x=dq.front();
dq.pop_front();
//cout<<"cac"<<endl;
gothere[x]=true;
for (auto c:neighbors[x]){
if (gothere[c]) continue;
if (component[c]!=component[x]){
dq.push_back(c);
d[c]=min(d[c], d[x]+1);
}else{
dq.push_front(c);
d[c]=min(d[c], d[x]);
}
}
}
for (int i=1; i<=temp; i++){
ans=max(ans, d[i]);
//cout<<d[i]<<endl;
}
cout<<ans;
}
