#include <bits/stdc++.h>
#define FR(i, N) for (int i = 0; i < int(N); i++)
#define all(x) begin(x), end(x)
using namespace std;
using ll = long long;
struct custom_hash {
static uint64_t splitmix64(uint64_t x) {
// http://xorshift.di.unimi.it/splitmix64.c
x += 0x9e3779b97f4a7c15;
x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
return x ^ (x >> 31);
}
size_t operator()(pair<uint64_t, uint64_t> x) const {
static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
return splitmix64(x.first + FIXED_RANDOM) + splitmix64(x.second+FIXED_RANDOM);
}
};
const int MAXN = 4000 + 5;
int par[MAXN*MAXN];
int board[MAXN*MAXN];
int seen[MAXN*MAXN];
int rk[MAXN*MAXN];
int sz = 0;
int dist[MAXN*MAXN];
unordered_set<pair<int, int>, custom_hash> already;
vector<int> adj[int(4e6)];
int find(int x) {
if (par[x] == x) {
return x;
}
return par[x] = find(par[x]);
}
void merge(int x, int y) {
if (board[x] == board[y]) {
int pX = find(x);
int pY = find(y);
if (rk[pX] < rk[pY]) {
swap(pX, pY);
}
par[pY] = pX;
}
}
void addEdge(int x, int y) {
if (board[x] != board[y] && board[x] >= 0 && board[y] >= 0) {
pair<int, int> pos = {board[x], board[y]};
if (board[x] > board[y]) {
swap(pos.first, pos.second);
}
if (!already.count(pos)) {
already.insert(pos);
adj[board[x]].push_back(board[y]);
adj[board[y]].push_back(board[x]);
}
}
}
int H, W;
int conv(int i, int j) {
return i*W + j;
}
int main() {
cin.tie(0);
cin.sync_with_stdio(0);
cin >> H >> W;
W += 2;
H += 2;
FR(i, MAXN*MAXN) {
par[i] = i;
rk[i] = 1;
board[i] = '.';
}
FR(i, H-2) {
FR(j, W-2) {
char c;
cin >> c;
board[conv(i+1, j+1)] = c;
}
}
for (int i = 1; i < H-1; i++) {
for (int j = 1; j < W-1; j++) {
merge(conv(i, j), conv(i+1, j));
// merge(conv(i, j), conv(i-1, j));
merge(conv(i, j), conv(i, j+1));
// merge(conv(i, j), conv(i, j-1));
}
}
fill(all(seen), -1);
FR(i, MAXN*MAXN) {
if (board[i] != '.' && par[i] == i) {
seen[i] = sz++;
}
}
FR(i, MAXN*MAXN) {
board[i] = seen[find(i)];
}
for (int i = 1; i < H-1; i++) {
for (int j = 1; j < W-1; j++) {
addEdge(conv(i, j), conv(i+1, j));
// addEdge(conv(i, j), conv(i-1, j));
addEdge(conv(i, j), conv(i, j+1));
// addEdge(conv(i, j), conv(i, j-1));
}
}
fill(all(dist), MAXN*MAXN);
deque<int> p;
p.push_back(board[conv(1, 1)]);
dist[board[conv(1, 1)]] = 0;
int mx = 0;
while (p.size() > 0) {
int next = p.front();
p.pop_front();
for (int e : adj[next]) {
if (dist[e] > dist[next] + 1) {
dist[e] = dist[next]+1;
p.push_back(e);
mx = max(dist[e], mx);
}
}
}
cout << mx + 1 << '\n';
return 0;
}
