h,w = map(int,input().split())
field = [list(input())for i in range(h)]
a= [[0]*w for i in range(h)]

vec = [(-1,0),(1,0),(0,1),(0,-1)]
ans = 0
for i in range(h):
    for j in range(w):
        if field[i][j] == "#":continue
        stack = [(i,j)]
        visited = [[0]*w for i in range(h)]
        a[i][j] = 0
        
        while stack:
            next = []
            for x,y in stack:
                
                if visited[x][y]:continue

                visited[x][y] = 1
                

                for tx,ty in vec:
                    nx = x+tx
                    ny = y+ty
                    if nx < 0 or ny <0 or nx >=h or ny >=w:continue
                    if field[nx][ny] == "#" :continue
                    if visited[nx][ny]:continue
                    next.append((nx,ny))
                    a[nx][ny] = min(a[x][y] + 1,a[nx][ny])
                    
                    
            stack = next
   
        for i in range(h):
            for j in range(w):
                if a[i][j] != 10**4:
                    ans = max(ans,a[i][j])
                a[i][j] = 10**4
print(ans)
            
            
            
            