#include <bits/stdc++.h>
// Author: Kazuki_Will_Win_VOI_8703
#define fi first
#define se second
#define pii pair<int, int>
#define ll long long
#define all(a) a.begin(), a.end()
using namespace std;
const int mn = 4e3 + 5, mod = 998244353, inf = 1e9;
int h, w, d[mn][mn], vis[mn][mn];
char a[mn][mn];
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
bool Megumi(int i, int j){
if(i <= 0 || i >= h + 1 || j <= 0 || j >= w + 1) return false;
if(a[i][j] == '.' || vis[i][j]) return false;
return true;
}
void solve(){
cin >> h >> w;
for(int i = 1; i <= h; i++){
for(int j = 1; j <= w; j++){
cin >> a[i][j];
}
}
fill(&d[0][0], &d[0][0] + mn * mn, inf);
queue <pii> q1, q2;
d[1][1] = 1;
q1.push({1, 1});
int res = 0;
while(q1.size() || q2.size()){
if(!q1.size()) swap(q1, q2);
auto [i, j] = q1.front();
q1.pop();
res = max(res, d[i][j]);
for(int k = 0; k < 4; k++){
int ni = i + dx[k], nj = j + dy[k];
if(!Megumi(ni, nj)) continue;
vis[ni][nj] = true;
if(a[ni][nj] == a[i][j]){
if(d[ni][nj] > d[i][j]){
d[ni][nj] = d[i][j];
q1.push({ni, nj});
}
}
else{
if(d[ni][nj] > d[i][j] + 1){
d[ni][nj] = d[i][j] + 1;
q2.push({ni, nj});
}
}
}
}
cout << res << '\n';
}
signed main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int t = 1;
// cin >> t;
while(t--) solve();
}
