#include <bits/stdc++.h>
#define FOR(i, x, y) for (int i = x; i < y; i++)
typedef long long ll;
using namespace std;
int n, m, component[4000][4000], ans = 1;
char snow[4000][4000];
int num = 1;
unordered_set<int> graph[8000050];
int visited[8000050], q[8000050];
void floodfill(int c, int x, int y) {
component[x][y] = c;
if (x > 0 && snow[x - 1][y] != '.') {
if (snow[x - 1][y] == snow[x][y] && !component[x - 1][y]) floodfill(c, x - 1, y);
else if (component[x - 1][y]) {
graph[c].insert(component[x - 1][y]);
graph[component[x - 1][y]].insert(c);
}
}
if (x < n - 1 && snow[x + 1][y] != '.') {
if (snow[x + 1][y] == snow[x][y] && !component[x + 1][y]) floodfill(c, x + 1, y);
else if (component[x + 1][y]) {
graph[c].insert(component[x + 1][y]);
graph[component[x + 1][y]].insert(c);
}
}
if (y > 0 && snow[x][y - 1] != '.') {
if (snow[x][y - 1] == snow[x][y] && !component[x][y - 1]) floodfill(c, x, y - 1);
else if (component[x][y - 1]) {
graph[c].insert(component[x][y - 1]);
graph[component[x][y - 1]].insert(c);
}
}
if (y < m - 1 && snow[x][y + 1] != '.') {
if (snow[x][y + 1] == snow[x][y] && !component[x][y + 1]) floodfill(c, x, y + 1);
else if (component[x][y + 1]) {
graph[c].insert(component[x][y+ 1]);
graph[component[x][y + 1]].insert(c);
}
}
}
int main() {
iostream::sync_with_stdio(false);
cin.tie(0);
cin >> n >> m;
FOR(i, 0, n) FOR(j, 0, m) cin >> snow[i][j];
FOR(i, 0, n) FOR(j, 0, m) {
if (!component[i][j] && snow[i][j] != '.') {
floodfill(num++, i, j);
}
}
int rp = -1, lp = 0;
q[++rp] = 1;
visited[1] = 1;
while (rp >= lp) {
int curr = q[lp++];
for (int i : graph[curr]) {
if (!visited[i]) {
visited[i] = visited[curr] + 1;
ans = max(ans, visited[i]);
q[++rp] = i;
}
}
}
cout << ans;
return 0;
}
