보물섬

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.PriorityQueue;
import java.util.StringTokenizer;
 
public class Main {
    static int n, m, ans = 0;
    static int map[][] = new int[51][51];
    static boolean visited[][] = new boolean[51][51];
    static int dp[][] = new int[51][51];
    static final int INF = 987654321;
    static int dx[] = {-1,1,0,0}, dy[] = {0,0,-1,1};
 
    public static void main(String[] args) throws IOException{
        // TODO Auto-generated method stub
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st= new StringTokenizer(br.readLine());
 
        n = Integer.parseInt(st.nextToken()); //물건의 개수 
        m = Integer.parseInt(st.nextToken()); //비교쌍의 개수 
 
        for (int i = 0; i < n; i++) {
            String s = br.readLine();
            for (int j = 0; j < m; j++) {
                char c = s.charAt(j);
                if (c == 'L')
                    map[i][j] = 1;
                else map[i][j] = 0;
            }
        }
 
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                dp[i][j] = INF;
            }
        }
 
        //        for (int i = 0; i < n; i++) {
        //            for (int j = 0; j < m; j++) {
        //                System.out.print(map[i][j] + " ");
        //            }
        //            System.out.println();
        //        }
 
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if (map[i][j] == 1) {
                    dijkstra(i, j);
                    calc();
                }
            }
        }
 
        //        for (int i = 0; i < n; i++) {
        //            for (int j = 0; j < m; j++) {
        //                if (dp[i][j] == INF) System.out.print("- ");
        //                else System.out.print(dp[i][j] + " ");
        //            }
        //            System.out.println();
        //        }
        System.out.println(ans);
 
    }
    static void calc() {
        //         System.out.println();
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
//                                if (dp[i][j] == INF) System.out.print("- ");
//                                else System.out.print(dp[i][j] + " ");
                if (dp[i][j] != INF)
                    ans = ans < dp[i][j] ? dp[i][j] : ans;
                dp[i][j] = INF;
                visited[i][j] = false;
            }
//                        System.out.println();
        }
//                 System.out.println();
    }
 
    static void dijkstra(int x, int y) {
        PriorityQueue<Pos> q = new PriorityQueue<>();
 
        q.add(new Pos(x, y, 0));
 
        dp[x][y] = 0;
 
        while(!q.isEmpty()) {
            int cx = q.peek().x;
            int cy = q.peek().y;
            q.poll();
 
            if (visited[cx][cy]) continue;
            visited[cx][cy] = true;
 
            for (int k = 0; k < 4; k++) {
                int nx = cx + dx[k];
                int ny = cy + dy[k];
 
                if (nx < 0 || nx >= n || ny < 0 || ny >= m || visited[nx][ny] || map[nx][ny] == 0) continue;
 
                if (dp[nx][ny] > dp[cx][cy] + 1) {
                    dp[nx][ny] = dp[cx][cy] + 1;
                    q.add(new Pos(nx, ny, dp[nx][ny]));
                }
            }
        }
    }
 
    static class Pos implements Comparable<Pos>{
        int x, y;
        int dist;
 
        public Pos(int x, int y, int dist) {
            this.x =x;
            this.y = y;
            this.dist = dist;
        }
 
        @Override
        public int compareTo(Pos o) {
            // TODO Auto-generated method stub
            if (this.dist > o.dist) return 1;
            else if (this.dist < o.dist) return -1;
            else return 0;
        }
    }
}