#include <bits/stdc++.h> // 0/1 BFS
using namespace std;
#define MAXN 4001
string g[MAXN];
int n, m, depth[MAXN][MAXN], dx[4] = { -1, 0, 1, 0 }, dy[4] = { 0, 1, 0, -1 };
bool inside(int i, int j) {
return i >= 0 && i < n && j >= 0 && j < m && g[i][j] != '.';
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
cin >> n >> m;
for (int i = 0; i < n; ++i)
cin >> g[i];
depth[0][0] = 1; //depth[i][j] = 0 means unvisited
deque<pair<int, int>> q;
q.push_back({ 0, 0 });
int ans = 1;
while (!q.empty()) {
auto z = q.front();
q.pop_front();
ans = max(ans, depth[z.first][z.second]);
for (int i = 0; i < 4; ++i) {
int f = z.first + dx[i], s = z.second + dy[i];
if (inside(f, s) && depth[f][s] == 0) {
if (g[f][s] == g[z.first][z.second]) {
depth[f][s] = depth[z.first][z.second];
q.push_front({ f, s });
}
else {
depth[f][s] = depth[z.first][z.second] + 1;
q.push_back({ f, s });
}
}
}
}
cout << ans;
}
