#include <bits/stdc++.h>
using namespace std;
const int dy[4]={0, 1, 0, -1};
const int dx[4]={1, 0, -1, 0};
int main() {
cin.tie(nullptr);
ios::sync_with_stdio(false);
int h, w;
cin >> h >> w;
vector<vector<char>> mat(h+2, vector<char>(w+2, '.'));
vector<vector<int>> breadth(h+2, vector<int>(w+2, 0));
for(int i=1;i<=h;i++) for(int j=1;j<=w;j++) cin >> mat[i][j];
breadth[1][1]=1;
vector<pair<int, int>> perb[2];
perb[0].push_back({1, 1});
for(int cur=0;!perb[cur].empty(); cur^=1) {
for(int i=0;i<perb[cur].size();i++) {
auto pr=perb[cur][i];
int y=pr.first, x=pr.second;
for(int k=0;k<4;k++) {
int newy=y+dy[k];
int newx=x+dx[k];
if(!breadth[newy][newx]&&mat[newy][newx]==mat[y][x]) {
breadth[newy][newx]=breadth[y][x];
perb[cur].push_back({newy, newx});
}
}
}
for(auto pr : perb[cur]) {
int y=pr.first, x=pr.second;
for(int k=0;k<4;k++) {
int newy=y+dy[k];
int newx=x+dx[k];
if(!breadth[newy][newx]&&mat[newy][newx]!='.') {
breadth[newy][newx]=breadth[y][x]+1;
perb[cur^1].push_back({newy, newx});
}
}
}
perb[cur].clear();
}
int ans=0;
for(int i=1;i<=h;i++) for(int j=1;j<=w;j++) ans=max(ans, breadth[i][j]);
cout << ans << endl;
}
