#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef short int shit;
#define ff first
#define ss second
bool check(int n, int m, int y, int x){
return x>=0&&x<m&&y>=0&&y<n;
}
int main(){
ios_base::sync_with_stdio(false), cin.tie(NULL);
// freopen("odometer.in", "r", stdin);
// freopen("odometer.out", "w", stdout);
int n, m;
cin >> n >> m;
vector<vector<char>> plain(n, vector<char>(m));
vector<vector<int>> depth(n, vector<int>(m, 0));
for(int i = 0; i < n; ++i){
for(int j = 0; j < m; ++j){
cin >> plain[i][j];
}
}
int dy[] = {-1,0,1,0};
int dx[] = {0,1,0,-1};
int ans = 1;
deque<pair<int,int>> dq;
dq.push_back({0,0});
depth[0][0] = 1;
while (!dq.empty()){
pair<int,int> coords = dq.front(); dq.pop_front();
ans = max(ans, depth[coords.ff][coords.ss]);
for(int i = 0; i < 4; ++i){
int yy = coords.ff+dy[i];
int xx = coords.ss+dx[i];
if(check(n,m,yy,xx)){
if(plain[yy][xx]!=plain[coords.ff][coords.ss]&plain[yy][xx]!='.'&&depth[yy][xx]==0){
dq.push_back({yy,xx});
depth[yy][xx] = depth[coords.ff][coords.ss] + 1;
}else if(plain[yy][xx]==plain[coords.ff][coords.ss]&plain[yy][xx]!='.'&&depth[yy][xx]==0){
dq.push_front({yy,xx});
depth[yy][xx] = depth[coords.ff][coords.ss];
}
}
}
}
cout << ans << '\n';
return 0;
}
