Buggy python submission (verdict: Runtime Error):
import sys
def input():
    return sys.stdin.readline()[:-1]
h,w=map(int,input().split())
s=[input() for i in range(h)]
# print(40**3)

def warshall_floyd(d):
    #d[i][j]: iからjへの最短距離
    for k in range(n):
        for i in range(n):
            for j in range(n):
                d[i][j] = min(d[i][j],d[i][k] + d[k][j])
    return d

# n,w = map(int,input().split()) #n:頂点数　w:辺の数
n=h*w
d = [[float("inf") for i in range(h*w)] for i in range(h*w)]
# print(d)
#d[u][v] : 辺uvのコスト(存在しないときはinf)
for i in range(h):
    for j in range(w):
        if i+1<h:
            if s[i][j]!="#" and s[i+1][j]!="#":
                d[i*h+j][(i+1)*h+j]=1
                d[(i+1)*h+j][i*h+j]=1
        if j+1<w:
            if s[i][j]!="#" and s[i][j+1]!="#":
                d[i*h+j][i*h+j+1]=1
                d[i*h+j+1][i*h+j]=1
# for i in range(w):
#     x,y,z = map(int,input().split())
#     d[x][y] = 1
#     d[y][x] = 1
for i in range(h*w):
    d[i][i] = 0 #自身のところに行くコストは０
li=warshall_floyd(d)
ans=0
for i in range(len(li)):
    for j in range(len(li[i])):
        if li[i][j]!=float("inf"):
            ans=max(ans,li[i][j])

print(ans)

Fixed submission:
import sys
def input():
    return sys.stdin.readline()[:-1]
h,w=map(int,input().split())
s=[input() for i in range(h)]
# print(40**3)

def warshall_floyd(d):
    #d[i][j]: iからjへの最短距離
    for k in range(n):
        for i in range(n):
            for j in range(n):
                d[i][j] = min(d[i][j],d[i][k] + d[k][j])
    return d

# n,w = map(int,input().split()) #n:頂点数　w:辺の数
n=h*w
d = [[float("inf") for i in range(h*w)] for i in range(h*w)]
# print(d)
#d[u][v] : 辺uvのコスト(存在しないときはinf)
for i in range(h):
    for j in range(w):
        if i+1<h:
            if s[i][j]!="#" and s[i+1][j]!="#":
                d[i*w+j][(i+1)*w+j]=1
                d[(i+1)*w+j][i*w+j]=1
        if j+1<w:
            if s[i][j]!="#" and s[i][j+1]!="#":
                d[i*w+j][i*w+j+1]=1
                d[i*w+j+1][i*w+j]=1
# for i in range(w):
#     x,y,z = map(int,input().split())
#     d[x][y] = 1
#     d[y][x] = 1
for i in range(h*w):
    d[i][i] = 0 #自身のところに行くコストは０
# print(d)
li=warshall_floyd(d)
ans=0
for i in range(len(li)):
    for j in range(len(li[i])):
        if li[i][j]!=float("inf"):
            ans=max(ans,li[i][j])

print(ans)

Buggy python submission (verdict: Runtime Error):
import sys
def input():
    return sys.stdin.readline()[:-1]
h,w=map(int,input().split())
s=[input() for i in range(h)]
print(40**3)

def warshall_floyd(d):
    #d[i][j]: iからjへの最短距離
    for k in range(n):
        for i in range(n):
            for j in range(n):
                d[i][j] = min(d[i][j],d[i][k] + d[k][j])
    return d

# n,w = map(int,input().split()) #n:頂点数　w:辺の数
n=h*w
d = [[float("inf") for i in range(h*w)] for i in range(h*w)]
# print(d)
#d[u][v] : 辺uvのコスト(存在しないときはinf)
for i in range(h):
    for j in range(w):
        if i+1<h:
            if s[i][j]!="#" and s[i+1][j]!="#":
                d[i*h+j][(i+1)*h+j]=1
                d[(i+1)*h+j][i*h+j]=1
        if j+1<w:
            if s[i][j]!="#" and s[i][j+1]!="#":
                d[i*h+j][i*h+j+1]=1
                d[i*h+j+1][i*h+j]=1
# for i in range(w):
#     x,y,z = map(int,input().split())
#     d[x][y] = 1
#     d[y][x] = 1
for i in range(h*w):
    d[i][i] = 0 #自身のところに行くコストは０
li=warshall_floyd(d)
ans=0
for i in range(len(li)):
    for j in range(len(li[i])):
        if li[i][j]!=float("inf"):
            ans=max(ans,li[i][j])

print(ans)    

Fixed submission:
import sys
def input():
    return sys.stdin.readline()[:-1]
h,w=map(int,input().split())
s=[input() for i in range(h)]
# print(40**3)

def warshall_floyd(d):
    #d[i][j]: iからjへの最短距離
    for k in range(n):
        for i in range(n):
            for j in range(n):
                d[i][j] = min(d[i][j],d[i][k] + d[k][j])
    return d

# n,w = map(int,input().split()) #n:頂点数　w:辺の数
n=h*w
d = [[float("inf") for i in range(h*w)] for i in range(h*w)]
# print(d)
#d[u][v] : 辺uvのコスト(存在しないときはinf)
for i in range(h):
    for j in range(w):
        if i+1<h:
            if s[i][j]!="#" and s[i+1][j]!="#":
                d[i*w+j][(i+1)*w+j]=1
                d[(i+1)*w+j][i*w+j]=1
        if j+1<w:
            if s[i][j]!="#" and s[i][j+1]!="#":
                d[i*w+j][i*w+j+1]=1
                d[i*w+j+1][i*w+j]=1
# for i in range(w):
#     x,y,z = map(int,input().split())
#     d[x][y] = 1
#     d[y][x] = 1
for i in range(h*w):
    d[i][i] = 0 #自身のところに行くコストは０
# print(d)
li=warshall_floyd(d)
ans=0
for i in range(len(li)):
    for j in range(len(li[i])):
        if li[i][j]!=float("inf"):
            ans=max(ans,li[i][j])

print(ans)