#include <bits/stdc++.h>
/*
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4")
*/
#define ll long long
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define IOS ios_base::sync_with_stdio(0);
using namespace std;
const ll maxn=2e5+123,inf=1e18,mod=1e9+7,N=1e7+12,LOG=20;
int n,m,ans;
pair<int,int> dir[]={{0,-1},{-1,0},{1,0},{0,1}};
bool was[4001][4001];
string s[4001];
bool check(int x,int y){
if(x<0 || x>=n || y<0 || y>=m || s[x][y]=='.')
return 0;
return 1;
}
int main(){
#ifdef LOCAL
freopen ("test.in", "r", stdin);
#endif
cin>>n>>m;
for(int i=0;i<n;i++)
cin>>s[i];
queue< pair<int,int> > q1,q2;
q1.push({0,0});
was[0][0]=1;
while(true){
while(!q1.empty()){
int x=q1.front().f,y=q1.front().s;
//cout<<x<<" "<<y<<endl;
q1.pop();
for(int i=0;i<4;i++){
int nx=x+dir[i].f,ny=y+dir[i].s;
if( check(nx,ny) && !was[nx][ny] ){
if(s[x][y]==s[nx][ny])
q1.push({nx,ny});
else
q2.push({nx,ny});
was[nx][ny]=1;
}
}
}
ans++;
if(q2.empty())
break;
swap(q1,q2);
}
cout<<ans;
}
