#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
const int maxn = 4001;
int dr[4] = {-1, 0, 1, 0};
int dc[4] = {0, 1, 0, -1};
int R, C;
char grid[maxn][maxn];
inline bool in(int r, int c) {
return r >= 0 && r < R && c >= 0 && c < C;
}
int numcc;
int ccid[maxn][maxn];
vector<vector<int>> adj;
void dfs(int r, int c) {
char was = grid[r][c];
grid[r][c] = '.';
ccid[r][c] = numcc;
for (int d = 0; d < 4; ++d) {
int r1 = r + dr[d], c1 = c + dc[d];
if (in(r1, c1) && grid[r1][c1] == was) {
dfs(r1, c1);
}
}
}
int maxd;
bitset<maxn*maxn> visited;
void dfs2(int u, int d) {
maxd = max(maxd, d);
visited[u] = true;
for (int v : adj[u]) {
if (visited[v] == false) {
dfs2(v, d+1);
}
}
}
int main() {
ios::sync_with_stdio(false); cin.tie(0);
cin >> R >> C;
for (int r = 0; r < R; ++r) {
for (int c = 0; c < C; ++c) {
cin >> grid[r][c];
}
}
numcc = 0;
for (int r = 0; r < R; ++r) {
for (int c = 0; c < C; ++c) {
if (grid[r][c] != '.') {
dfs(r, c);
++numcc;
}
}
}
adj.resize(numcc);
for (int r = 0; r < R; ++r) {
for (int c = 0; c < C; ++c) {
for (int d = 0; d < 4; ++d) {
int r1 = r + dr[d], c1= c + dc[d];
if (in(r1, c1) && ccid[r1][c1] != ccid[r][c]) {
adj[ccid[r][c]].push_back(ccid[r1][c1]);
adj[ccid[r1][c1]].push_back(ccid[r][c]);
}
}
}
}
maxd = 0;
dfs2(0, 0);
cout << maxd+1 << endl;
}
