// https://oj.uz/problem/view/BOI13_tracks
#include <cstdio>
#include <algorithm>
#include <functional>
#include <vector>
#include <cstring>
#include <queue>
#include <set>
using namespace std;
typedef long long ll;
// n,m <= 4000
int h, w;
char s[4005][4005];
int c[4005][4005];
set<int> adj[4005];
queue<pair<int,int>> q;
void floodfill(int i, int j, char t, int co) {
q.push({i,j});
while (!q.empty()) {
tie(i, j) = q.front();
q.pop();
if (i < 0 || i >= h || j < 0 || j >= w || c[i][j]) continue;
if (s[i][j] != t) continue;
c[i][j] = co;
q.push({i+1,j});
q.push({i-1,j});
q.push({i,j+1});
q.push({i,j-1});
}
}
void link(int i, int j, int p, int q) {
if (p < 0 || p >= h || q < 0 || q >= w) return;
if (s[i][j] == '.' || s[p][q] == '.') return;
int c1 = c[i][j], c2 = c[p][q];
if (c1 == c2) return;
adj[c1].insert(c2);
adj[c2].insert(c1);
}
queue<pair<int,int>> Q;
vector<int> d;
int main() {
scanf("%d %d", &h, &w);
for (int i = 0; i < h; i++)
scanf("%s", s[i]);
int co = 1;
// floodfill
for (int i = 0; i < h; i++)
for (int j = 0; j < w; j++) {
if (s[i][j] != '.' && c[i][j] == 0) {
floodfill(i, j, s[i][j], co++);
}
}
// link the nodes
for (int i = 0; i < h; i++)
for (int j = 0; j < w; j++) {
link(i,j,i+1,j);
link(i,j,i-1,j);
link(i,j,i,j+1);
link(i,j,i,j-1);
}
// bfs to find depth from (1,1)
Q.push({c[0][0],1});
d.resize(co);
for (int i = 0; i < co; i++)
d[i] = 0;
int ans = 0;
while (!Q.empty()) {
int u = Q.front().first;
int dep = Q.front().second;
Q.pop();
if (d[u]) continue;
d[u] = dep;
ans = max(ans, dep);
for (int v: adj[u]) {
Q.push({v, dep+1});
}
}
printf("%d", ans);
return 0;
}
