#include <bits/stdc++.h>
using namespace std;
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<string> a(n);
for (string &s : a) cin >> s;
vector<vector<int>> d(n, vector<int>(m, INT_MAX));
deque<pair<int, int>> dq;
dq.emplace_front(0, 0);
d[0][0] = 1;
int dx[4] = {0, 0, -1, 1};
int dy[4] = {-1, 1, 0, 0};
while (!dq.empty()) {
auto [x, y] = dq.front();
dq.pop_front();
for (int k = 0; k < 4; k++) {
int r = x + dx[k], c = y + dy[k];
if (min({r, c, n - r - 1, m - c - 1}) >= 0) {
if (a[r][c] == '.') continue;
if (a[r][c] == a[x][y]) {
int nd = d[x][y];
if (nd < d[r][c]) {
d[r][c] = nd;
dq.emplace_front(r, c);
}
}
else {
int nd = d[x][y] + 1;
if (nd < d[r][c]) {
d[r][c] = nd;
dq.emplace_back(r, c);
}
}
}
}
}
int ans = 0;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if (a[i][j] != '.') ans = max(ans, d[i][j]);
}
}
cout << ans;
return 0;
}
