/*input
5 8
FFR.....
.FRRR...
.FFFFF..
..RRRFFR
.....FFF
*/
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
const int N = 1666661;
const int dx[] = {0, 0, 1, -1}, dy[] = {1, -1, 0, 0};
int Arr[N], Size[N], visit[N], l[N], n, m, ans = 1;
char Type[N];
vector<int> child[N];
int get(int x, int y){
int k = (x - 1) * (m + 1) + y;
return (0 <= k && k < N) ? k : 0;
}
void init(){
for(int i = 0; i < N; ++i)
Arr[i] = i, Size[i] = 1;
}
int root(int a){
if(a == Arr[a]) return a;
return Arr[a] = root(Arr[a]);
}
void Union(int a, int b){
a = root(a), b = root(b);
if(a == b) return;
if(Size[a] < Size[b]) swap(a, b);
Arr[b] = a, Size[a] += Size[b];
}
int main(){
memset(visit, -1, sizeof visit);
init();
scanf("%d%d", &n, &m);
for(int i = 1; i <= n; ++i)
for(int j = 1; j <= m; ++j){
char ch; scanf(" %c", &ch);
if(ch == '.') continue;
Type[get(i, j)] = ch;
visit[get(i, j)] = 0;
}
for(int i = 1; i <= n; ++i)
for(int j = 1; j <= m; ++j)
if(visit[get(i, j)] != -1) for(int dir = 0; dir < 4; ++dir){
int newx = i + dx[dir], newy = j + dy[dir];
if(visit[get(newx, newy)] == -1 || Type[get(i, j)] != Type[get(newx, newy)]) continue;
Union(get(i, j), get(newx, newy));
}
for(int i = 1; i <= n; ++i)
for(int j = 1; j <= m; ++j)
if(visit[get(i, j)] != -1) for(int dir = 0; dir < 4; ++dir){
int newx = i + dx[dir], newy = j + dy[dir];
if(visit[get(newx, newy)] == -1 || Type[get(i, j)] == Type[get(newx, newy)]) continue;
int u = root(get(newx, newy)), v = root(get(i, j));
child[u].push_back(v), child[v].push_back(u);
}
memset(visit, 0, sizeof visit);
queue<int> q;
q.push(root(1));
visit[root(1)] = 1;
while(q.size()){
int u = q.front(); q.pop();
sort(child[u].begin(), child[u].end());
child[u].erase(unique(child[u].begin(), child[u].end()), child[u].end());
for(int v:child[u])
if(!visit[v]){
l[v] = l[u] + 1, ans = max(ans, l[v] + 1);
visit[v] = 1, q.push(v);
}
}
// printf("%d\n", root(1));
// for(int i = 1; i <= n; ++i){
// for(int j = 1; j <= m; ++j)
// printf("%d ", root(get(i, j)));
// puts("");
// }
printf("%d\n", ans);
}
