#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const int MaxN = 4e3 + 5;
int h, w, n;
char arr[MaxN][MaxN];
void Inp()
{
cin >> h >> w;
for (int x = 1; x <= h; x++)
{
for (int y = 1; y <= w; y++)
{
cin >> arr[x][y];
}
}
n = h * w;
}
int Hash(int a, int b)
{
return (a - 1) * w + b;
}
int parents[MaxN * MaxN];
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, -1, 1};
int FindSet(int p)
{
if (parents[p] == p)
{
return p;
}
parents[p] = FindSet(parents[p]);
return parents[p];
}
void UnionSet(int a, int b)
{
a = FindSet(a);
b = FindSet(b);
if (a == b)
{
return;
}
parents[b] = a;
}
bool CheckX(int a)
{
return 1 <= a && a <= h;
}
bool CheckY(int a)
{
return 1 <= a && a <= w;
}
vector<int> graph[MaxN * MaxN];
queue<int> dq;
int F[MaxN * MaxN];
void Exc()
{
for (int x = 1; x <= n; x++)
{
parents[x] = x;
}
for (int x = 1; x <= h; x++)
{
for (int y = 1; y <= w; y++)
{
if (arr[x][y] == '.')
{
continue;
}
for (int z = 0; z < 4; z++)
{
int newx = x + dx[z], newy = y + dy[z];
if (CheckX(newx) && CheckY(newy) && arr[newx][newy] == arr[x][y])
{
UnionSet(Hash(newx, newy), Hash(x, y));
}
}
}
}
for (int x = 1; x <= h; x++)
{
for (int y = 1; y <= w; y++)
{
if (arr[x][y] == '.')
{
continue;
}
for (int z = 0; z < 4; z++)
{
int newx = x + dx[z], newy = y + dy[z];
if (CheckX(newx) && CheckY(newy) && arr[newx][newy] != '.' && arr[newx][newy] != arr[x][y])
{
graph[FindSet(Hash(newx, newy))].push_back(FindSet(Hash(x, y)));
}
}
}
}
dq.push(FindSet(1));
F[FindSet(1)] = 1;
int res = 0;
while (!dq.empty())
{
int u = dq.front();
dq.pop();
res = max(res, F[u]);
for (int v : graph[u])
{
if (!F[v])
{
F[v] = F[u] + 1;
dq.push(v);
}
}
}
cout << res;
}
int main()
{
//freopen("B.INP", "r", stdin);
//freopen("B.OUT", "w", stdout);
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int test = 1;
//cin >> test;
for (int x = 1; x <= test; x++)
{
Inp();
Exc();
}
return 0;
}
