#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
#define all(x) x.begin(), x.end()
#define rep(i, l, n) for (int i = l; i < (n); ++i)
#define sz(x) (int)x.size()
const char nl = '\n';
const int mod = 1e9+7;
const int N = 4001;
int a[N*N], vis[N*N];
set<int> g[N*N];
int find(int x) {
if (a[x] == x)return x;
return a[x] = find(a[x]);
}
void unite(int x, int y) {
x = find(x), y = find(y);
a[x] = y;
}
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};
int c[N][N];
int n, m;
bool can(int i, int j) {
if (i < 0 || i >= n || j < 1 || j > m)return false;
return true;
}
int res = 0;
void solve() {
cin >> n >> m;
vector<string> s(n);
rep(i, 0, n*m+1)a[i] = i;
rep(i, 0, n) {
cin >> s[i];
s[i] = '&'+s[i];
}
rep(i, 0, n)
rep(j, 1, m+1)
rep(k, 0, 4) {
int x = i+dx[k], y = j+dy[k];
if (can(x, y) && s[x][y] != '.' && s[x][y] == s[i][j])unite(i*m+j, x*m+y);
}
int cnt = 0;
vector<int> nd(n*m+10);
rep(i, 0, n)
rep(j, 1, m+1) {
if (s[i][j] == '.')continue;
int x = find(i*m+j);
if (vis[x])continue;
nd[x] = ++cnt, vis[x] = 1;
}
rep(i, 0, n)
rep(j, 1, m+1)
rep(k, 0, 4) {
int x = i+dx[k], y = j+dy[k];
if (can(x, y) && s[x][y] != '.' && s[x][y] != s[i][j] && s[i][j] != '.') {
x = find((i+dx[k])*m+j+dy[k]), y = find(i*m+j);
g[nd[x]].insert(nd[y]);
}
}
memset(vis, 0, sizeof vis);
queue<P> q;
q.push(P(1, 1));
vis[1] = 1;
while (!q.empty()) {
auto &[x, y] = q.front(); q.pop();
res = max(res, y);
for (auto i: g[x])
if (!vis[i]) {
vis[i] = 1;
q.push({i, y+1});
}
}
cout << res << nl;
}
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
