#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define pf push_front
#define tiiic tuple<int, int, int, char>
const int maxn = 4005;
const int inf = 2e9;
const int di[] = {0, 0, 1, -1};
const int dj[] = {-1, 1, 0, 0};
int n, m;
char grid[maxn][maxn];
deque<tiiic> dq;
int res = -inf;
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 >> grid[i][j];
}
}
dq.pb({0, 0, 1, grid[0][0]});
grid[0][0] = '.';
while(!dq.empty()){
auto [r, c, l, t] = dq.front();
dq.pop_front();
res = max(res, l);
for(int i=0;i<4;++i){
int vr = r + di[i], vc = c + dj[i];
if(vr < 0 || vc < 0 || vr >= n || vc >= m || grid[vr][vc] == '.') continue;
if(grid[vr][vc] == t){
dq.pf({vr, vc, l, t});
grid[vr][vc] = '.';
}
else{
dq.pb({vr, vc, l + 1, grid[vr][vc]});
grid[vr][vc] = '.';
}
}
}
cout << res;
}
