#include <bits/stdc++.h>
#define int long long
#define vi vector<int>
#define pb push_back
#define sz(a) a.size()
#define pii pair<int,int>
#define fi first
#define se second
#define all(a) a.begin(), a.end()
using namespace std;
const int MAXN=4005;
char G [MAXN][MAXN];
int dp [MAXN][MAXN];
vector<pii> disp {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
void solve () {
int n,m;
cin>>n>>m;
for(int i=1;i<=n;i++) {
string s;
cin>>s;
fill(&dp[i][0], &dp[i][0]+m+1, 1e9);
for(int j=1;j<=m;j++)
G[i][j]=s[j-1];
}
//0-1 bfs as this grid only has 1-weight edges
deque<pii> Q;
Q.pb({1,1});
dp[1][1]=1;
int ans=0;
while(!Q.empty()) {
pii v=Q.front();
Q.pop_front();
for(auto [dy, dx]:disp) {
pii to {v.fi+dy, v.se+dx};
if(to.fi<=0||to.se<=0||to.fi>n||to.se>m||G[to.fi][to.se]=='.')
continue;
if(dp[to.fi][to.se]==1e9) {
if(G[to.fi][to.se]==G[v.fi][v.se]) {
dp[to.fi][to.se]=dp[v.fi][v.se];
Q.push_front(to);
}else {
dp[to.fi][to.se]=dp[v.fi][v.se]+1;
Q.push_back(to);
}
ans=max(ans, dp[to.fi][to.se]);
}
}
}
cout<<ans<<"\n";
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int t=1;
// cin>>t;
while(t>0) {
solve();
t--;
}
}
