#include <bits/stdc++.h>

using namespace std;
int l,w,ans;
char board[52][52];
int dx[4] = {-1,0,1,0};
int dy[4] = {0,-1,0,1};

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);

    cin >> l >> w;
    for(int i=0; i<l; ++i)
        for(int j=0; j<w; ++j)
            cin >> board[i][j];
    queue<pair<int,int>> q;
    for(int i=0; i<l; ++i)
        for(int j=0; j<w; ++j){
            if(board[i][j] == 'W') continue;
            int dist[52][52] = {0,};
            dist[i][j] = 1;
            q.push({i,j});
            while(!q.empty()){
                auto cur = q.front(); q.pop();
                ans = max(ans,dist[cur.first][cur.second]);
                for(int dir=0; dir<4; ++dir){
                    int nx = cur.first + dx[dir];
                    int ny = cur.second + dy[dir];
                    if(nx<0||ny<0||nx>=l||ny>=w) continue;
                    if(board[nx][ny] == 'W' || dist[nx][ny]) continue;
                    dist[nx][ny] = dist[cur.first][cur.second] + 1;
                    q.push({nx,ny});
                }
            }
        }
    cout << ans-1;
}