#include <algorithm>
#include <array>
#include <deque>
#include <ios>
#include <iostream>
#include <string>
#include <vector>
using std::array;
using std::cin;
using std::cout;
using std::deque;
using std::ios_base;
using std::max;
using std::max_element;
using std::string;
using std::vector;
using Location = array<int, 2>;
constexpr auto dir_1 = array<int, 4>({1, 0, -1, 0});
constexpr auto dir_2 = array<int, 4>({0, 1, 0, -1});
auto solve() {
auto h = 0;
auto w = 0;
cin >> h >> w;
auto map = vector<string>(h);
for (auto& x : map) {
cin >> x;
}
auto distances = vector<vector<int>>(h, vector<int>(w, -1));
distances[0][0] = 0;
auto parents = vector<vector<Location>>(h, vector<Location>(w, Location({-1, -1})));
auto process = deque<Location>({Location({0, 0})});
while (!process.empty()) {
const auto front = process.front();
process.pop_front();
const auto& [row, col] = front;
const auto [parent_row, parent_col] = parents[row][col];
if (row || col) {
distances[row][col] = (
distances[parent_row][parent_col] + (map[row][col] != map[parent_row][parent_col])
);
}
for (auto i = 0; i < 4; ++i) {
const auto new_row = row + dir_1[i];
const auto new_col = col + dir_2[i];
if (
new_row >= 0 && new_row < h && new_col >= 0 && new_col < w &&
parents[new_row][new_col][0] == -1 && (new_row || new_col)
) {
parents[new_row][new_col] = front;
if (map[new_row][new_col] == map[row][col]) {
process.push_front(Location({new_row, new_col}));
} else {
process.push_back(Location({new_row, new_col}));
}
}
}
}
auto animals = 0;
for (const auto& x : distances) {
animals = max(*max_element(x.begin(), x.end()), animals);
}
cout << animals << '\n';
}
int main() {
cin.tie(nullptr);
ios_base::sync_with_stdio(false);
solve();
return 0;
}
