#include <iostream>
#include <deque>
#include <vector>
#include <limits>
using namespace std;
int maxY, maxX;
//bool visited[4000][4000] = {false};
int distances[4000][4000];
vector<string> grid;
pair<int, int> add(pair<int, int> a, pair<int, int> b){
return make_pair(a.first+b.first,a.second+b.second);
}
bool valid(pair<int, int> a){
if (a.first<0||a.first>=maxX||a.second<0||a.second>=maxY){
return false;
}
return grid[a.second][a.first]!='.';
}
int getEdge(pair<int, int> a, pair<int, int> b){
return grid[a.second][a.first] == grid[b.second][b.first] ? 0 : 1;
}
string s;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> maxY >> maxX;
for (int i = 0; i < maxY; i++){
cin >> s;
grid.push_back(s);
for (int j = 0; j < maxX; j++){
distances[i][j] = numeric_limits<int>::max()/2;
}
}
//visited[0][0] = false;
deque<pair<int, int>> currents;
int best = 0;
distances[0][0] = 0;
currents.emplace_back(0, 0);
while (currents.size()>0){
pair<int, int> current = currents.front();
currents.pop_front();
best = max(distances[current.second][current.first], best);
pair<int, int> nextOne;
nextOne = add(current, make_pair(1,0));
if (valid(nextOne)){
int newDistance = distances[current.second][current.first] + getEdge(current, nextOne);
if (newDistance < distances[nextOne.second][nextOne.first]){
distances[nextOne.second][nextOne.first] = newDistance;
if (getEdge(current, nextOne)==0){
currents.push_front(nextOne);
}
else{
currents.push_back(nextOne);
}
}
}
nextOne = add(current, make_pair(-1,0));
if (valid(nextOne)){
int newDistance = distances[current.second][current.first] + getEdge(current, nextOne);
if (newDistance < distances[nextOne.second][nextOne.first]){
distances[nextOne.second][nextOne.first] = newDistance;
if (getEdge(current, nextOne)==0){
currents.push_front(nextOne);
}
else{
currents.push_back(nextOne);
}
}
}nextOne = add(current, make_pair(0,1));
if (valid(nextOne)){
int newDistance = distances[current.second][current.first] + getEdge(current, nextOne);
if (newDistance < distances[nextOne.second][nextOne.first]){
distances[nextOne.second][nextOne.first] = newDistance;
if (getEdge(current, nextOne)==0){
currents.push_front(nextOne);
}
else{
currents.push_back(nextOne);
}
}
}nextOne = add(current, make_pair(0,-1));
if (valid(nextOne)){
int newDistance = distances[current.second][current.first] + getEdge(current, nextOne);
if (newDistance < distances[nextOne.second][nextOne.first]){
distances[nextOne.second][nextOne.first] = newDistance;
if (getEdge(current, nextOne)==0){
currents.push_front(nextOne);
}
else{
currents.push_back(nextOne);
}
}
}
}
cout << best + 1;
}
