#include <cstdio>
#include <queue>
using namespace std;
int height, width;
char map[51][51];
int bfs(int starty, int startx)
{
int visit[51][51] = { 0, };
int mx = 0;
int dy[4] = { -1, 0, 0, 1 };
int dx[4] = { 0, -1, 1, 0 };
queue<pair<int, int> > que;
visit[starty][startx] = 1;
que.push(make_pair(starty, startx));
while (!que.empty()) {
int y = que.front().first;
int x = que.front().second;
que.pop();
if (visit[y][x] > mx) mx = visit[y][x];
for (int i = 0; i < 4; i++) {
int nexty = y + dy[i];
int nextx = x + dx[i];
if (nexty >= 0 && nexty < height && nextx >= 0 && nextx < width) {
if (map[nexty][nextx] == 'L' && visit[nexty][nextx] == 0) {
que.push(make_pair(nexty, nextx));
visit[nexty][nextx] = visit[y][x] + 1;
}
}
}
}
return mx - 1;
}
int solve()
{
int mx = 0, val;
for (int i = 0; i < height; i++) {
for (int j = 0; j < width; j++) {
if (map[i][j] == 'L') {
val = bfs(i, j);
if (val > mx) mx = val;
}
}
}
return mx;
}
int main()
{
scanf("%d %d", &height, &width);
for (int i = 0; i < height; i++) {
scanf("%s", map[i]);
}
printf("%d\n", solve());
return 0;
}