/*
ID: swagchicken1
PROG:
LANG: C++11
*/
#include <iostream>
#include <tuple>
#include <cmath>
#include <string>
#include <cstring>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <algorithm>
#include <vector>
#include <fstream>
#include <iomanip>
#include <ctime>
#include <cctype>
#include <climits>
#include <chrono>
#include <numeric>
#include <functional>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector< vector <int> > vvi;
typedef pair<int, int> pii;
typedef pair < pair < int, int >, int > piii;
typedef pair < pair <int, int > , pair <int, int> > piiii;
typedef pair<ll, ll> pll;
typedef vector<bool> vb;
typedef vector<char> vc;
typedef vector<string> vs;
#define FOR(i,a,b) for(int i = a; i < b; i ++)
#define RFOR(i,a,b) for(int i = a-1; i >= b; i --)
#define all(a) a.begin(), a.end()
#define endl '\n';
#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define ff first
#define ss second
template <typename T>
void pr(vector<T> &v) {
FOR(i, 0, sz(v)) cout << v[i] << " ";
cout << endl;
}
template <typename T>
void pr(vector<vector<T> > &v) {
FOR(i, 0, sz(v)) { pr(v[i]); }
}
template <typename T>
void re(T &x) {
cin >> x;
}
template <typename T>
void re(vector<T> &a) {
FOR(i, 0, sz(a)) re(a[i]);
}
template <class Arg, class... Args>
void re(Arg &first, Args &... rest) {
re(first);
re(rest...);
}
template <typename T>
void pr(T x) {
cout << x << endl;
}
template <class Arg, class... Args>
void pr(const Arg &first, const Args &... rest) {
cout << first << " ";
pr(rest...);
cout << endl;
}
void ps() { cout << endl; }
template<class T, class... Ts>
void ps(const T& t, const Ts&... ts) {
cout << t; if (sizeof...(ts)) cout << " "; ps(ts...);
}
const ll MOD = 1000000007;
#define inf 1e18;
#define INF INT_MAX
//#define DEBUG
long double PI = 4*atan(1);
long double eps = 1e-9;
char g[4010][4010] = {};
int dist[4010][4010] = {};
int main() {
//auto start = chrono::high_resolution_clock::now();
ios_base::sync_with_stdio(0);cin.tie(0);
//ofstream cout("output.txt");
//ifstream cin("input.txt");
#ifdef DEBUG
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
// freopen("pieaters.in", "r", stdin);
// freopen("pieaters.out", "w", stdout);
#endif
int n,m; cin >> n >> m;
FOR(i,0,n) {
FOR(j,0,m) {
cin >> g[i][j];
dist[i][j] = 1e9;
}
}
int dr[4] = {1,-1,0,0};
int dc[4] = {0,0,1,-1};
deque<pair<pii, int>> dq;
dist[0][0] = 0;
dq.push_back({{0,0}, 0});
int ans = 0;
while(!dq.empty()) {
pair<pii, int> curr = dq.front();
pii x = curr.ff;
int d = 0;
dq.pop_front();
FOR(k,0,4) {
int nr = x.ff + dr[k];
int nc = x.ss + dc[k];
if(nr >= 0 && nr < n && nc >= 0 && nc < m) {
if(dist[nr][nc] == 1e9) {
int nd = d + (g[x.ff][x.ss] != g[nr][nc]);
dist[nr][nc] = nd;
ans = max(ans, nd);
if(nd == d) {
dq.push_front({{nr, nd}, nd});
} else {
dq.push_back({{nr, nd}, nd});
}
}
}
}
}
cout << ans + 1 << endl;
//auto stop = chrono::high_resolution_clock::now();
//auto duration = chrono::duration_cast<chrono::microseconds>(stop - start);
//cout << duration.count() << endl;
//cin.close();
//cout.close();
}
