// IN THE NAME OF GOD
#include<bits/stdc++.h>
#define endl '\n'
#define file_reading freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout);
#define flush cout.flush();
using namespace std;
typedef long long ll;
typedef long double dll;
typedef unsigned long long ull;
const int N = 4e3+5;
int dat[N][N], mark[N][N], ans, having, must;
vector<pair<int,int>> gone, later;
int n,m;
void dfs(int x, int y){
mark[x][y] = 1;
having++;
if (x > 1 && dat[x][y] == dat[x-1][y] && !mark[x-1][y]) dfs(x-1,y);
if (x < n && dat[x][y] == dat[x+1][y] && !mark[x+1][y]) dfs(x+1,y);
if (y > 1 && dat[x][y] == dat[x][y-1] && !mark[x][y-1]) dfs(x,y-1);
if (y < m && dat[x][y] == dat[x][y+1] && !mark[x][y+1]) dfs(x,y+1);
if (x > 1 && dat[x][y] != dat[x-1][y] && !mark[x-1][y] && dat[x][y] + dat[x-1][y] == 3) later.push_back({x-1,y});
if (x < n && dat[x][y] != dat[x+1][y] && !mark[x+1][y] && dat[x][y] + dat[x+1][y] == 3) later.push_back({x+1,y});
if (y > 1 && dat[x][y] != dat[x][y-1] && !mark[x][y-1] && dat[x][y] + dat[x][y-1] == 3) later.push_back({x,y-1});
if (y < m && dat[x][y] != dat[x][y+1] && !mark[x][y+1] && dat[x][y] + dat[x][y+1] == 3) later.push_back({x,y+1});
}
void solve(){
cin >> n >> m;
for(int i=1; i<=n; i++){
for(int j=1; j<=m; j++){
char g; cin >> g;
if (g == '.') dat[i][j] = 0;
else if (g == 'R') dat[i][j] = 1, must++;
else dat[i][j] = 2, must++;
}
}
if (must == 0){cout << 0 << endl; return;}
ans = 1;
dfs(n,m);
while(having != must){
ans++;
gone.clear();
for(pair<int,int> cur : later) gone.push_back(cur);
later.clear();
for(pair<int,int> cur : gone) if (!mark[cur.first][cur.second]) dfs(cur.first,cur.second);
}
cout << ans << endl;
}
int32_t main(){
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int t; t=1; //cin >> t;
while(t--){solve();}
}
