#include <bits/stdc++.h>
using namespace std;
const int N = 4000;
string grid[N];
bool vu[N][N];
bitset<N*N> seen;
int comp[N][N];
vector<int> adj[N*N];
int h,w;
vector<pair<int,int>> dir = {{1,0},{0,1},{-1,0},{0,-1}};
void dfs(int x, int y, int k, char c){
if (x < 0 or y < 0 or x >= h or y >= w or vu[x][y] or grid[x][y] != c) return;
vu[x][y] = true;
comp[x][y] = k;
for (auto d:dir){
int nx = d.first+x, ny = d.second+y;
dfs(nx,ny,k,c);
}
}
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
cin >> h >> w;
for (int i=0; i<h; ++i){
cin >> grid[i];
}
int k = 0;
for (int i=0; i<h; ++i){
for (int j=0; j<w; ++j){
if (!vu[i][j]){
dfs(i,j,k,grid[i][j]);
++k;
}
}
}
for (int i=0; i<h; ++i){
for (int j=0; j<w; ++j){
for (auto d:dir){
int nx = d.first+i, ny = d.second+j;
if (nx >= 0 and ny >= 0 and nx < h and ny < w and comp[nx][ny] != comp[i][j]){
adj[comp[nx][ny]].emplace_back(comp[i][j]);
adj[comp[i][j]].emplace_back(comp[nx][ny]);
}
}
}
}
deque<pair<int,int>> q;
q.emplace_back(0,comp[0][0]);
int ans = 0;
while (!q.empty()){
auto f = q.front();
q.pop_front();
ans = max(ans,f.first);
for (auto i:adj[f.second]){
if (!seen[i]){
seen[i] = true;
q.emplace_back(f.first+1,i);
}
}
}
cout << ans;
return 0;
}
