#include "bits/stdc++.h"
using namespace std;
#define ln "\n"
#define pb push_back
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define int long long
const int MOD = 1e9 + 7;
const int inf = 2e18;
const int N = 4005;
int n, m, a[N][N], dist[N][N];
bool block[N][N];
pair<int, int> d[5] = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
signed main() {
cin.tie(0) -> sync_with_stdio(0);
cin >> n >> m;
deque<pair<int, int>> q;
for(int i = 1; i <= n; i++) {
for(int j = 1; j <= m; j++) {
char c; cin >> c;
if(c == '.') block[i][j] = 1;
else if(c == 'R') a[i][j] = 1;
else a[i][j] = 2;
}
}
dist[1][1] = 1;
int ans = 0;
q.push_front({1, 1});
while(sz(q)) {
auto u = q.front(); q.pop_front();
int x = u.fi, y = u.se;
block[x][y] = 1;
ans = max(ans, dist[x][y]);
for(int i = 0; i < 4; i++) {
int nx = x + d[i].fi, ny = y + d[i].se;
if(nx < 1 or ny < 1 or nx > n or ny > m or block[nx][ny]) continue;
if(a[nx][ny] == a[x][y]) {
dist[nx][ny] = dist[x][y];
q.push_front({nx, ny});
} else {
dist[nx][ny] = dist[x][y] + 1;
q.push_back({nx, ny});
}
}
}
cout << ans << ln;
cerr << "\nTime: " << clock() * 1000 / CLOCKS_PER_SEC;
}
