#include <bits/stdc++.h>
using namespace std;
using i64 = int64_t;
using u64 = uint64_t;
using u32 = uint32_t;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int h,w;
cin >> h >> w;
vector<vector<int>> depths(h, vector<int>(w, -1));
vector<string> meadow(h);
for(auto& str : meadow)
cin >> str;
deque<pair<int, int>> qu;
int ans = 0;
const int xoff[4] = {-1, 0, 0, 1};
const int yoff[4] = {0, -1, 1, 0};
depths[0][0] = 0;
qu.push_back({0, 0});
while(qu.size()) {
auto [y, x] = qu.front();
qu.pop_front();
ans = max(ans, depths[y][x]);
for(int cy,cx,i=0; i<4; i++) {
cy = y + yoff[i];
cx = x + xoff[i];
if(cy < 0 || cx < 0 || cy >= h || cx >= w || depths[cy][cx] >= 0)
continue;
if(meadow[y][x] == meadow[cy][cx]) {
qu.push_front({cy, cx});
depths[cy][cx] = depths[y][x];
}
else {
qu.push_back({cy, cx});
depths[cy][cx] = depths[y][x]+1;
}
}
}
cout << ans << '\n';
return 0;
}
