#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;
#define X first
#define Y second
const int MXN = 4001;
int n, m;
char a[MXN][MXN];
int dis[MXN][MXN];
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, 1, 0, -1};
int32_t main() {
cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
cin >> n >> m;
for(int i=0; i<n; i++)
for(int j=0; j<m; j++)
cin >> a[i][j],
dis[i][j] = -1;
deque<pii> dq;
dq.push_back({0, 0});
dis[0][0] = 1;
while(!dq.empty()) {
auto [x, y] = dq.front();
dq.pop_front();
for(int d=0, xx, yy; d<4; d++) {
xx = x + dx[d], yy = y + dy[d];
if(xx<0 || xx>=n || yy<0 || yy>=m || a[xx][yy]=='.' || dis[xx][yy]!=-1) continue;
if(a[x][y]==a[xx][yy]) {
dis[xx][yy] = dis[x][y];
dq.push_front({xx, yy});
}
else {
dis[xx][yy] = dis[x][y]+1;
dq.push_back({xx, yy});
}
}
}
int ans=0;
for(int i=0; i<n; i++)
for(int j=0; j<m; j++)
if(a[i][j]!='.')
ans = max(ans, dis[i][j]);
cout << ans << '\n';
return 0;
}
