#include <bits/stdc++.h>
using namespace std;
struct dir{
int vt,hz;
}a[8]={{0,1},{0,-1},{1,0},{1,-0}};
int h,w,ct,mv,ans,dis[4005][4005],len,x,y,newx,newy;
char s[4005][4005],c;
bool vis[4005][4005];
priority_queue <pair<int,pair<pair<int,int>,char> > > q;
int main(){
ios::sync_with_stdio(false);
cin >> h >> w;
for (int i=1;i<=h;i++){
for (int j=1;j<=w;j++){
cin >> s[i][j];
if (s[i][j] != '.')
ct++;
}
}
q.push(make_pair(-1,make_pair(make_pair(1,1),s[1][1])));
while (!q.empty()){
x=q.top().second.first.first; y=q.top().second.first.second;
len=-q.top().first; c=q.top().second.second;
q.pop();
if (!vis[x][y]){
vis[x][y]=true; dis[x][y]=len;
for (int i=0;i<4;i++){
newx=x+a[i].vt; newy=y+a[i].hz;
if (newx > 0 && newx <= h && newy > 0 && newy <= w && !vis[newx][newy] && s[newx][newy] != '.'){
if (c != s[newx][newy])
q.push(make_pair(-len-1,make_pair(make_pair(newx,newy),s[newx][newy])));
else
q.push(make_pair(-len,make_pair(make_pair(newx,newy),s[newx][newy])));
}
}
}
}
for (int i=1;i<=h;i++){
for (int j=1;j<=w;j++){
ans=max(ans,dis[i][j]);
}
}
cout << ans << "\n";
return 0;
}
