#include <bits/stdc++.h>
#define N 4003
#define f first
#define s second
using namespace std;
typedef pair<int, int> pii;
int n, m, ans, cnt, ok[N][N], dx[4] = {0, 0, 1, -1}, dy[4] = {1, -1, 0, 0};
int id[N][N], pai[N*N], dist[N*N], peso[N*N];
inline int Find(int x)
{
if(pai[x] == x) return x;
return pai[x] = Find(pai[x]);
}
inline void join(int a, int b)
{
a = Find(a), b = Find(b);
if(a == b) return;
if(peso[a] > peso[b]) pai[b] = a;
else if(peso[a] < peso[b]) pai[a] = b;
else pai[a] = b, peso[b] ++;
}
char mat[N][N], T;
vector<int> grafo[N*N];
int main()
{
ios::sync_with_stdio(false); cin.tie(0);
cin>>n>>m;
for(int i = 1; i <= n; i++)
for(int j = 1; j <= m; j++)
cin>>mat[i][j];
for(int i = 1; i <= n; i++)
for(int j = 1; j <= m; j++)
id[i][j] = ++cnt;
for(int i = 1; i <= cnt; i++) pai[i] = i;
for(int i = 1; i <= n; i++)
{
for(int j = 1; j <= m; j++)
{
if(mat[i][j] == '.') continue;
for(int p = 0; p < 4; p++)
{
int a = i + dx[p], b = j + dy[p];
if(!a or !b or a > n or b > m or mat[a][b] != mat[i][j]) continue;
join(id[a][b], id[i][j]);
//cout<<"JOIN "<<a<<" "<<b<<" "<<i<<" "<<j<<"\n";
}
}
}
for(int i = 1; i <= n; i++)
{
for(int j = 1; j <= m; j++)
{
if(mat[i][j] == '.') continue;
for(int p = 0; p < 4; p++)
{
int a = i + dx[p], b = j + dy[p];
if(!a or !b or a > n or b > m) continue;
if(mat[a][b] != mat[i][j] and mat[a][b] != '.')
{
int u = Find(id[a][b]), v = Find(id[i][j]);
grafo[u].push_back(v);
grafo[v].push_back(u);
}
}
}
}
queue<int> fila;
memset(dist, 0x3f3f3f3f, sizeof dist);
dist[Find(id[1][1])] = 1;
fila.push(Find(id[1][1]));
while(!fila.empty())
{
int x = fila.front();
fila.pop();
for(auto v: grafo[x])
{
if(dist[v] > dist[x] + 1)
{
dist[v] = dist[x] + 1;
fila.push(v);
}
}
}
/*for(int i = 1; i <= n; i++)
{
for(int j = 1; j <= m; j++)
{
cout<<Find(id[i][j])<<" ";
}
cout<<"\n";
}*/
for(int i = 1; i <= cnt; i++)
{
//cout<<i<<" "<<dist[i]<<"\n";
if(dist[i] != 0x3f3f3f3f) ans = max(ans, dist[i]);
}
cout<<ans<<"\n";
}
