#include <bits/stdc++.h>
#define maxn 4005
#define fi first
#define se second
using namespace std;
using ii = pair<int, int>;
int n, m;
char a[maxn][maxn];
int cl[maxn][maxn];
int dv[4] = {0, 1, 0, -1};
int dh[4] = {1, 0, -1, 0};
int par[maxn*maxn];
int nho[maxn*maxn], id = 0;
int find_set(int v) {
return par[v] < 0 ? v : par[v] = find_set(par[v]);
}
void union_set(int u, int v) {
u = find_set(u); v = find_set(v);
if (u == v) return;
if (par[u] < par[v]) swap(u, v);
par[v] += par[u];
par[u] = v;
}
bool ok(int i, int j) {
return (1 <= i && i <= n && 1 <= j && j <= m);
}
int rem[maxn*maxn];
vector<int> adj[maxn*maxn];
void bfs(int ox, int oy) {
queue<ii> q;
q.push(ii{ox, oy});
cl[ox][oy] = ++id;
vector<int> nodes;
while (!q.empty()) {
auto [u, v] = q.front(); q.pop();
for (int k = 0; k < 4; k++) {
int i = u + dv[k], j = v + dh[k];
if (!ok(i, j)) continue;
if (a[i][j] != a[u][v] && a[i][j] != '.') {
if (cl[i][j] && !rem[cl[i][j]]) {
rem[cl[i][j]] = 1;
adj[cl[i][j]].emplace_back(id);
adj[id].emplace_back(cl[i][j]);
nodes.emplace_back(cl[i][j]);
}
} else if (a[i][j] == a[u][v] && !cl[i][j]) {
cl[i][j] = id;
q.push(ii{i, j});
}
}
}
for (int i : nodes) rem[i] = 0;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m;
for (int i = 1; i <= n; i++) for (int j = 1; j <= m; j++) cin >> a[i][j];
fill(par+1, par+n*m+1, -1);
for (int i = 1; i <= n; i++)
for (int j = 1; j <= m; j++)
if (a[i][j] != '.' && !cl[i][j]) bfs(i, j);
queue<int> q;
nho[1] = 1;
q.push(1);
while (!q.empty()) {
int u = q.front(); q.pop();
for (int v : adj[u])
if (!nho[v]) {
nho[v] = nho[u]+1;
q.push(v);
}
}
cout << *max_element(nho + 1, nho + id + 1);
}
