#include<bits/stdc++.h>
#include <bitset>
using namespace std;
const int N = 4000;
string S[2];
vector<pair<int,int>>Graf1[N*N]; //czy rozne kolory, wierzcholek
int idx[N*N], c=1;
bitset<N*N> c1, c2;
vector<int>Graf[N*N];
void DFS1(int n, int v){
c1[n] = c, idx[n] = v;
for(auto& elm : Graf1[n]){
if(c1[elm.second] == 0 && elm.first == 0){
DFS1(elm.second, v);
}
}
}
int makeTree(int n, int m){
int v = 0, d = 0;
for(int i=0;i<n*m;i++){
if(c1[i] == 0)
DFS1(i, v++);
}
for(int i=0;i<n*m;i++){
for(auto& elm : Graf1[i]){
if(idx[elm.second] != idx[i])
Graf[idx[i]].push_back(idx[elm.second]);
}
Graf1[i].clear();
}
queue<pair<int,int>>Q;
Q.push({0, 1});
while(!Q.empty()){
auto now = Q.front();
Q.pop();
if(c2[now.first] == c)
continue;
c2[now.first] = c, d = max(d, now.second);
for(auto& elm : Graf[now.first]){
if(c2[elm] != c){
Q.push({elm, now.second+1});
}
}
}
return d;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int n,m;
cin>>n>>m;
for(int i=0;i<n;i++){
S[0] = S[1];
cin>>S[1];
for(int j=0;j<m;j++){
if(S[1][j] == '.')
continue;
if(i != 0){
if(S[0][j] == S[1][j]){
Graf1[i*m+j].push_back({0, (i-1)*m+j}), Graf1[(i-1)*m+j].push_back({0, i*m+j});
}
else if(S[0][j] != '.'){
Graf1[i*m+j].push_back({1, (i-1)*m+j}), Graf1[(i-1)*m+j].push_back({1, i*m+j});
}
}
if(j != 0){
if(S[1][j-1] == S[1][j]){
Graf1[i*m+j-1].push_back({0, i*m+j}), Graf1[i*m+j].push_back({0, i*m+j-1});
}
else if(S[1][j-1] != '.'){
Graf1[i*m+j-1].push_back({1, i*m+j}), Graf1[i*m+j].push_back({1, i*m+j-1});
}
}
}
}
cout<<makeTree(n, m)<<"\n";
}
