#include <bits/stdc++.h>
using namespace std;
#define int long long
#define eb emplace_back
#define mp make_pair
#define pb push_back
#define pp pop_back
#define endl '\n'
#define ff first
#define ss second
#define stop exit(0)
#define sz(x) (int)x.size()
#define pause system("pause")
#define all(x) (x).begin(), (x).end()
#define deb(x) cout << #x << "-" << x << endl
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
typedef char chr;
typedef string str;
typedef long long ll;
typedef vector<int> vii;
typedef pair<int, int> pii;
const long long INF = LLONG_MAX;
const int inf = INT_MAX;
const int mod = 998244353;
const int MOD = 1000000007;
const int dx[] = {-1,0,1,0};
const int dy[] = {0,1,0,-1};
const double PI = 2 * acos(0.0);
const int N = 4e3+5;
int h,w;
string bd[N];
int cnt;
int d[N][N];
bool inbound(int x, int y) {
return (1 <= x && x <= h && 1 <= y && y <= w);
}
inline void test_case() {
cin >> h >> w;
for (int i = 1; i <= h; i++) {
cin >> bd[i];
bd[i] = " " + bd[i];
for (int j = 1; j <= w; j++) {
d[i][j] = inf;
}
}
queue<pair<pii,int>> q;
q.push({{1,1},1});
d[1][1]=1;
int cur = 0;
while (!q.empty()) {
auto [x,y] = q.front().ff;
int c = q.front().ss;
q.pop();
if (d[x][y] != c) continue;
cur++;
for (int i = 0; i < 4; i++) {
int nx = x + dx[i];
int ny = y + dy[i];
if (!inbound(nx,ny)) continue;
if (d[nx][ny] > d[x][y]+(bd[nx][ny]!=bd[x][y])) {
d[nx][ny] = d[x][y]+(bd[nx][ny]!=bd[x][y]);
q.push({{nx,ny},d[nx][ny]});
}
}
}
int ans = -inf;
for (int i = 1; i <= h; i++) {
for (int j = 1; j <= w; j++) {
ans = max(ans,d[i][j]);
}
}
cout << ans << endl;
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int T = 1;
// cin >> T;
while (T--) test_case();
return 0;
}
