import queue
import itertools
r,c=map(int,input().split())
s=[list(input()) for i in range(r)]
ans=0
for i1 in range(r):
    for i2 in range(c):
        p=0
        a=[[s[j][i] for i in range(c)] for j in range(r)]
        sy,sx=i1,i2
        if a[sy][sx]=="#":
            break
                #print(sy,sx,gy,gx)
        q = queue.Queue()
        d=[(-1,0),(1,0),(0,-1),(0,1)]
        a[sy][sx]=0
        for i in d:
            if 0<=sy+i[0]<r and 0<=sx+i[1]<c and a[sy+i[0]][sx+i[1]]==".":
                a[sy+i[0]][sx+i[1]]=1
                q.put((sy+i[0],sx+i[1]))
        while not q.empty():
            y,x=q.get()
            for i in d:
                if 0<=y+i[0]<r and 0<=x+i[1]<c and a[y+i[0]][x+i[1]]==".":
                    p=a[y][x]+1
                    q.put((y+i[0],x+i[1]))
                ans=max(p,ans)
print(ans)

