#include <iostream>
#include <vector>
#include <deque>
#include <map>
using namespace std;
pair<int, int> dir[4] = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};
int main()
{
int h, w;
cin >> h >> w;
char meadow[h][w];
for (int i = 0; i < h; i++)
{
string s;
cin >> s;
for (int j = 0; j < w; j++) meadow[i][j] = s[j];
}
deque<pair<int, int>> q;
q.push_front({0, 0});
map<pair<int, int>, int> dist;
dist[{0, 0}] = 0;
int ans = 1;
while (!q.empty())
{
int i = q.front().first, j = q.front().second;
q.pop_front();
ans = max(ans, dist[{i, j}]);
for (auto &p : dir)
{
int inew = i + p.first, jnew = j + p.second;
if (inew < 0 || jnew < 0 || inew >= h || jnew >= w) continue;
if (dist.count({inew, jnew})) continue;
if (meadow[i][j] == meadow[inew][jnew])
{
q.push_front({inew, jnew});
dist[{inew, jnew}] = dist[{i, j}];
}
else
{
q.push_back({inew, jnew});
dist[{inew, jnew}] = dist[{i, j}] + 1;
}
}
}
cout << ans;
}
