#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int mxN = 4e3+10;
char matrix[mxN][mxN];
vector<int> xy = {1, 0, -1, 0}, yx = {0, 1, 0, -1};
int n, m, d[mxN][mxN];
bool valid(int a, int b) {
if(a < 0 || a >= n || b < 0 || b >= m ||
matrix[a][b] == '.') return false;
return true;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m;
for(int i = 0; i < n; i++) {
for(int j = 0; j < m; j++) {
cin >> matrix[i][j];
}
}
deque<array<int, 2>> q;
q.push_back({0, 0});
d[0][0] = 1;
int ans = 1;
while(!q.empty()) {
auto x = q.front()[0], y = q.front()[1];
q.pop_front();
ans = max(ans, d[x][y]);
for(int i = 0; i < 4; i++) {
int a = x + xy[i], b = y + yx[i];
if(valid(a, b) && d[a][b] == 0) {
if(matrix[a][b] == matrix[x][y]) {
d[a][b] = d[x][y];
q.push_front({a, b});
}
else {
d[a][b] = d[x][y]+1;
q.push_back({a, b});
}
}
}
}
cout << ans;
}
