#include <bits/stdc++.h>
#define ll long long
#define s second
#define f first
#define pb push_back
#define pii pair <int,int>
#define left (h<<1),l,(l + r)/2
#define right ((h<<1)|1),(l + r)/2 + 1,r
using namespace std;
const int N = 4000 + 3,inf = 1e9;
char a[N][N];
int d[N][N];
vector <pii> neighbours = {{0,1}, {0,-1}, {1,0}, {-1,0}};
signed main(){
ios_base::sync_with_stdio(0),cin.tie(NULL),cout.tie(NULL);
int n,m;
cin>>n>>m;
for (int i = 1; i <= n; i++)
for (int j = 1; j <= m; j++)
cin >> a[i][j],d[i][j] = inf;
priority_queue <pair<int,pii>> pq;
d[1][1] = 1;
pq.push({-1,{1,1}});
while (pq.size()){
int D = -pq.top().f,x = pq.top().s.f,y = pq.top().s.s;
pq.pop();
if (d[x][y] != D) continue;
for (auto [dx,dy]: neighbours){
int X = dx + x,Y = dy + y,w=0;
if (X < 1 || X > n || Y < 1 || Y > m) continue;
if (a[x][y] != a[X][Y]) w = 1;
if (d[X][Y] > D + w){
d[X][Y] = D + w;
pq.push({-D-w,{X,Y}});
}
}
}
int ans=0;
for (int i = 1; i <= n; i++)
for (int j = 1; j <= m; j++)
ans=max(ans,d[i][j]);
cout<<ans;
}
