#include <bits/stdc++.h>
using namespace std;
/*
ifstream fin("input.in");
ofstream fout("output.out");
*/
#define fin cin
#define fout cout
using ll = long long;
const int nmax = 4001;
struct point {int x,y;};
int n,m;
char mat[nmax][nmax];
int depth[nmax][nmax];
deque<point> dq;
const point d[] = {
{1,0},
{0,1},
{-1,0},
{0,-1}
};
bool inside(int x, int y) {
return 1 <= x and x <= n
and 1 <= y and y <= m
and mat[x][y] != '.';
}
int main() {
ios_base::sync_with_stdio(false);
fin >> n >> m;
for (int i = 1; i <= n; i++) for (int j = 1; j <= m; j++) fin >> mat[i][j];
dq.push_back({1,1});
depth[1][1] = 1;
int ans = 0;
while (!dq.empty()) {
auto [x,y] = dq.front();
dq.pop_front();
ans = max(ans,depth[x][y]);
for (auto [dx,dy] : d) {
int tx = dx + x;
int ty = dy + y;
if (inside(tx,ty) and !depth[tx][ty]) {
if (mat[x][y] == mat[tx][ty]) {
depth[tx][ty] = depth[x][y];
dq.push_front({tx,ty});
} else {
depth[tx][ty] = depth[x][y] + 1;
dq.push_back({tx,ty});
}
}
}
}
fout << ans;
return 0;
}
