#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <numeric>
#include <cmath>
#include <cstring>
using namespace std;
typedef long long ll;
int dx[4]{1, -1, 0, 0}, dy[4]{0, 0, 1, -1};
ll depth[4000][4000];
int main() {
ll h,w; cin >> h >> w;
ll map[h][w];
for (int i = 0; i< h; i++) {
for (int j = 0; j<w; j++) {
char k; cin >> k;
if (k=='.') map[i][j] = 0;
else if (k=='R') map[i][j] = 1;
else map[i][j] = 2;
}
}
deque<pair<ll,ll>> q;
q.push_back({0,0});
depth[0][0] = 1;
ll maxdepth = 1;
while (!q.empty()) {
ll x= q.front().first; ll y = q.front().second; q.pop_front();
for (int i = 0; i<4; i++) {
if (x+dx[i]<w && x+dx[i]>=0 && y+dy[i]<h && y+dy[i]>=0 && map[x+dx[i]][y+dy[i]]!=0) {
ll a = x+dx[i]; ll b = y+dy[i];
if (map[a][b]==map[x][y] && depth[a][b]==0) {
q.push_front({a,b});
depth[a][b] = depth[x][y];
}
else {
if (depth[a][b]==0 || depth[a][b] > depth[x][y]+1) {
depth[a][b] = depth[x][y]+1; q.push_back({a,b});
maxdepth = max(maxdepth,depth[a][b]);
}
}
}
}
}
cout << maxdepth;
}
