#include <bits/stdc++.h>
using namespace std;
#define MAX_N 4001
#define pb push_back
#define mp make_pair
#define f first
#define s second
#define int long long
using pi = pair<int, int>;
int n, m, curC = 1;
string grid[MAX_N];
int visited[MAX_N][MAX_N];
set<int> adj[MAX_N];
int distances[MAX_N];
void floodfill(int r, int c, char color) {
if(r < 0 || r >= n || c < 0 || c >= m || grid[r][c] == '.') return;
if(grid[r][c] != color && visited[r][c]) {
adj[curC-1].insert(visited[r][c]);
adj[visited[r][c]-1].insert(curC-1);
return;
}
else if(visited[r][c] || grid[r][c] != color) return;
visited[r][c] = curC;
floodfill(r+1, c, color);
floodfill(r-1, c, color);
floodfill(r, c+1, color);
floodfill(r, c-1, color);
}
void bfs(int node) {
queue<int> q;
q.push(node);
distances[node] = 1;
while(!q.empty()) {
int cur = q.front(); q.pop();
for(int i: adj[cur]) {
if(distances[i] == 0) {
distances[i] = distances[cur] + 1;
q.push(i);
}
}
}
}
signed main() {
cin >> n >> m;
for(int i = 0; i < n; ++i) cin >> grid[i];
for(int i = 0; i < n; ++i) {
for(int j = 0; j < m; ++j) {
if(visited[i][j] == 0 && grid[i][j] != '.') {
floodfill(i, j, grid[i][j]);
++curC;
}
}
}
// for(int i = 0; i < n; ++i) cout << grid[i] << endl;
// for(int i = 0; i < n; ++i) {for(int j = 0; j < m; ++j) cout << visited[i][j] << ' '; cout << endl;}
/* for(int i = 0; i < n; ++i) {
for(int j: adj[i]) cout << j << ' '; cout << endl;
} */
bfs(0);
int maxDist = 1;
for(int i = 0; i < n; ++i) {
maxDist = max(maxDist, distances[i]);
}
cout << maxDist << '\n';
return 0;
}
