h,w=map(int,input().split())
n=h*w
inf=10**6
field=[]
for i in range(h):
    field.append(input())
dist=[[inf]*n for i in range(n)]
for i in range(h):
    for j in range(w):
        if field[i][j]=="#":continue
        if j+1<w and field[i][j+1]!="#":
            dist[i*w+j][i*w+j+1]=1
            dist[i*w+j+1][i*w+j]=1
        if i+1<h and field[i+1][j]!="#":
            dist[i*w+j][(i+1)*w+j]=1
            dist[(i+1)*w+j][i*w+j]=1
for i in range(n):
    for j in range(n):
        for k in range(n):
            dist[j][k]=min(dist[j][k],dist[j][i]+dist[i][k])
ans=0
for i in range(n):
    for j in range(n):
        if i==j:
            continue
        if dist[i][j]==inf:
            continue
        ans=max(ans,dist[i][j])
print(ans)