#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using db = double;
template<typename T>
using v = vector<T>;
template<typename T, typename U>
using p = pair<T, U>;
#define rep(i, a, b) for (ll i = a; i < b; i++)
#define all(x) begin(x), end(x)
#define sz(x) (ll)(x).size()
ll n, m;
v<v<char>> grid;
v<v<ll>> num;
queue<p<ll, ll>> processing;
void dfs(ll row, ll col) {
if (row > 0 && grid[row][col] == grid[row - 1][col] && num[row - 1][col] == 1e18) {
processing.push({row - 1, col});
num[row - 1][col] = num[row][col];
dfs(row - 1, col);
}
if (row < n - 1 && grid[row][col] == grid[row + 1][col] && num[row + 1][col] == 1e18) {
processing.push({row + 1, col});
num[row + 1][col] = num[row][col];
dfs(row + 1, col);
}
if (col > 0 && grid[row][col] == grid[row][col - 1] && num[row][col - 1] == 1e18) {
processing.push({row, col - 1});
num[row][col - 1] = num[row][col];
dfs(row, col - 1);
}
if (col < m - 1 && grid[row][col] == grid[row][col + 1] && num[row][col + 1] == 1e18) {
processing.push({row, col + 1});
num[row][col + 1] = num[row][col];
dfs(row, col + 1);
}
}
void solve() {
cin >> n >> m;
grid = v<v<char>>(n, v<char>(m));
rep(i, 0, n) {
string s;
cin >> s;
rep(j, 0, m) {
grid[i][j] = s[j];
}
}
num = v<v<ll>>(n, v<ll>(m, 1e18));
num[0][0] = 1;
processing.push({0, 0});
dfs(0, 0);
while (!processing.empty()) {
p<ll, ll> fr = processing.front();
processing.pop();
ll row = fr.first; ll col = fr.second;
if (row > 0 && grid[row][col] != grid[row - 1][col] && grid[row - 1][col] != '.' && num[row - 1][col] == 1e18) {
num[row - 1][col] = num[row][col] + 1;
dfs(row - 1, col);
}
if (row < n - 1 && grid[row][col] != grid[row + 1][col] && grid[row + 1][col] != '.' && num[row + 1][col] == 1e18) {
num[row + 1][col] = num[row][col] + 1;
dfs(row + 1, col);
}
if (col > 0 && grid[row][col] != grid[row][col - 1] && grid[row][col - 1] != '.' && num[row][col - 1] == 1e18) {
num[row][col - 1] = num[row][col] + 1;
dfs(row, col - 1);
}
if (col < m - 1 && grid[row][col] != grid[row][col + 1] && grid[row][col + 1] != '.' && num[row][col + 1] == 1e18) {
num[row][col + 1] = num[row][col] + 1;
dfs(row, col + 1);
}
}
ll mx = 0;
rep(i, 0, n) {
rep(j, 0, m) {
//if (num[i][j] != 1e18) cout << num[i][j] << " ";
//else cout << "inf ";
if (num[i][j] != 1e18) mx = max(mx, num[i][j]);
} //cout << endl;
}
cout << mx << endl;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
ll t = 1;
//cin >> t;
while (t-- > 0)
solve();
}
