#include<bits/stdc++.h>
using namespace std;
#define ft first
#define sd second
int N,M,ans=0;
int depth[4005][4005];
string snow[4000];
int wx[5]={-1,0,0,1}, wy[5]={0,-1,1,0};
int check(int i,int j){
return (i>=0&&i<N&&j>=0&&j<M&&snow[i][j]!='.');
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> N >> M;
for(int i=0;i<N;i++) cin >> snow[i];
for(int i=0;i<N;i++){
for(int j=0;j<M;j++)
depth[i][j]=0;
}
deque<pair<int,int>> q;
q.push_back({0,0});
depth[0][0]=1;
while(!q.empty()){
int x=q.front().ft;
int y=q.front().sd;
q.pop_front();
ans=max(ans,depth[x][y]);
//cout << x << " " << y << endl;
for(int i=0;i<4;i++){
int nx=x+wx[i];
int ny=y+wy[i];
//cout << i << " : " << nx << " " << ny << endl;
//if(snow[nx][ny]=='.'){continue;}
if(check(nx,ny)&&depth[nx][ny]==0){
if(snow[x][y]==snow[nx][ny]){
depth[nx][ny]=depth[x][y];
q.push_front(make_pair(nx,ny));
}else{
depth[nx][ny]=depth[x][y]+1;
q.push_back(make_pair(nx,ny));
}
}
}
}
cout << ans;
}
