Buggy python submission (verdict: Runtime Error):
H,W=[int(i) for i in input().split(" ")]
Ch, Cw=[int(i)+1 for i in input().split(" ")]
Dh,Dw=[int(i)+1 for i in input().split(" ")]
S=[[False for i in range(W+4)],[False for i in range(W+4)]]
for i in range(H):
    S.append([False,False]+[j=='.' for j in input()]+[False,False])
S+=[[False for i in range(W+4)],[False for i in range(W+4)]]

def check(h,w,island_num):
    assert S[h][w]
    block[h][w]=island_num
    if S[h-1][w] and block[h-1][w]==-1:
        check(h-1,w,island_num)
    if S[h+1][w] and block[h+1][w]==-1:
        check(h+1,w,island_num)
    if S[h][w-1] and block[h][w-1]==-1:
        check(h,w-1,island_num)
    if S[h][w+1] and block[h][w+1]==-1:
        check(h,w+1,island_num)

block=[]
for i in range(H+4):
    block.append([-1 for j in range(W+4)])

island_num=0

for h in range(H+4):
    for w in range(W+4):
        if S[h][w] and block[h][w]==-1:
            check(h,w,island_num)
            island_num+=1


neighbor={}
for island in range(island_num):
    neighbor[island]=set()


for h in range(2,H+2):
    for w in range(2,W+2):
        if block[h][w]!=-1:
            for y in range(h-2,h+3):
                for x in range(w-2, w+3):
                    if block[y][x]!=-1 and block[h][w]!=block[y][x]:
                        neighbor[block[h][w]].add(block[y][x])


start=block[Ch][Cw]
goal=block[Dh][Dw]
assert start!=-1
assert goal!=-1



dist=[10**10 for i in range(island_num)]
dist[start]=0

nexts=[start]
while len(nexts)!=0:
    for n in nexts:
        _nexts=[]
        for neigh in neighbor[n]:
            assert neigh!=-1
            if dist[neigh]>dist[n]+1:
                _nexts.append(neigh)
                dist[neigh]=dist[n]+1
        nexts=set(_nexts)

if dist[goal]==10**10:
    print(-1)
else:
    print(dist[goal])
    assert dist[goal]==-1


Fixed submission:
H,W=[int(i) for i in input().split(" ")]
Ch, Cw=[int(i)+1 for i in input().split(" ")]
Dh,Dw=[int(i)+1 for i in input().split(" ")]
S=[[False for i in range(W+4)],[False for i in range(W+4)]]
for i in range(H):
    S.append([False,False]+[j=='.' for j in input()]+[False,False])
S+=[[False for i in range(W+4)],[False for i in range(W+4)]]

def check(h,w,island_num):
    assert S[h][w]
    block[h][w]=island_num
    if S[h-1][w] and block[h-1][w]==-1:
        check(h-1,w,island_num)
    if S[h+1][w] and block[h+1][w]==-1:
        check(h+1,w,island_num)
    if S[h][w-1] and block[h][w-1]==-1:
        check(h,w-1,island_num)
    if S[h][w+1] and block[h][w+1]==-1:
        check(h,w+1,island_num)

block=[]
for i in range(H+4):
    block.append([-1 for j in range(W+4)])

island_num=0

for h in range(H+4):
    for w in range(W+4):
        if S[h][w] and block[h][w]==-1:
            check(h,w,island_num)
            island_num+=1


neighbor={}
for island in range(island_num):
    neighbor[island]=set()


for h in range(2,H+2):
    for w in range(2,W+2):
        if block[h][w]!=-1:
            for y in range(h-2,h+3):
                for x in range(w-2, w+3):
                    if block[y][x]!=-1 and block[h][w]!=block[y][x]:
                        neighbor[block[h][w]].add(block[y][x])


start=block[Ch][Cw]
goal=block[Dh][Dw]
assert start!=-1
assert goal!=-1



dist=[10**10 for i in range(island_num)]
dist[start]=0

nexts=[start]
while True:
    _nexts=[]
    for n in nexts:
        for neigh in neighbor[n]:
            assert neigh!=-1
            if dist[neigh]>dist[n]+1:
                _nexts.append(neigh)
                dist[neigh]=dist[n]+1
    nexts=set(_nexts)
    if len(nexts)==0:
        break

if dist[goal]==10**10:
    print(-1)
else:
    print(dist[goal])

Buggy python submission (verdict: Runtime Error):
H,W=[int(i) for i in input().split(" ")]
Ch, Cw=[int(i)+1 for i in input().split(" ")]
Dh,Dw=[int(i)+1 for i in input().split(" ")]
S=[[False for i in range(W+4)],[False for i in range(W+4)]]
for i in range(H):
    S.append([False,False]+[j=='.' for j in input()]+[False,False])
S+=[[False for i in range(W+4)],[False for i in range(W+4)]]

def check(h,w,island_num):
    assert S[h][w]
    block[h][w]=island_num
    if S[h-1][w] and block[h-1][w]==-1:
        check(h-1,w,island_num)
    if S[h+1][w] and block[h+1][w]==-1:
        check(h+1,w,island_num)
    if S[h][w-1] and block[h][w-1]==-1:
        check(h,w-1,island_num)
    if S[h][w+1] and block[h][w+1]==-1:
        check(h,w+1,island_num)

block=[]
for i in range(H+4):
    block.append([-1 for j in range(W+4)])

island_num=0

for h in range(H+4):
    for w in range(W+4):
        if S[h][w] and block[h][w]==-1:
            check(h,w,island_num)
            island_num+=1


neighbor={}
for island in range(island_num):
    neighbor[island]=set()


for h in range(2,H+2):
    for w in range(2,W+2):
        if block[h][w]!=-1:
            for y in range(h-2,h+3):
                for x in range(w-2, w+3):
                    if block[y][x]!=-1 and block[h][w]!=block[y][x]:
                        neighbor[block[h][w]].add(block[y][x])


start=block[Ch][Cw]
goal=block[Dh][Dw]
assert start!=-1
assert goal!=-1



dist=[10**10 for i in range(island_num)]
dist[start]=0

nexts=[start]
while len(nexts)!=0:
    for n in nexts:
        _nexts=[]
        for neigh in neighbor[n]:
            assert neigh!=-1
            if dist[neigh]>dist[n]+1:
                _nexts.append(neigh)
                dist[neigh]=dist[n]+1
        nexts=set(_nexts)

if dist[goal]==10**10:
    print(-1)
    assert dist[goal]==-1
else:
    print(dist[goal])


Fixed submission:
H,W=[int(i) for i in input().split(" ")]
Ch, Cw=[int(i)+1 for i in input().split(" ")]
Dh,Dw=[int(i)+1 for i in input().split(" ")]
S=[[False for i in range(W+4)],[False for i in range(W+4)]]
for i in range(H):
    S.append([False,False]+[j=='.' for j in input()]+[False,False])
S+=[[False for i in range(W+4)],[False for i in range(W+4)]]

def check(h,w,island_num):
    assert S[h][w]
    block[h][w]=island_num
    if S[h-1][w] and block[h-1][w]==-1:
        check(h-1,w,island_num)
    if S[h+1][w] and block[h+1][w]==-1:
        check(h+1,w,island_num)
    if S[h][w-1] and block[h][w-1]==-1:
        check(h,w-1,island_num)
    if S[h][w+1] and block[h][w+1]==-1:
        check(h,w+1,island_num)

block=[]
for i in range(H+4):
    block.append([-1 for j in range(W+4)])

island_num=0

for h in range(H+4):
    for w in range(W+4):
        if S[h][w] and block[h][w]==-1:
            check(h,w,island_num)
            island_num+=1


neighbor={}
for island in range(island_num):
    neighbor[island]=set()


for h in range(2,H+2):
    for w in range(2,W+2):
        if block[h][w]!=-1:
            for y in range(h-2,h+3):
                for x in range(w-2, w+3):
                    if block[y][x]!=-1 and block[h][w]!=block[y][x]:
                        neighbor[block[h][w]].add(block[y][x])


start=block[Ch][Cw]
goal=block[Dh][Dw]
assert start!=-1
assert goal!=-1



dist=[10**10 for i in range(island_num)]
dist[start]=0

nexts=[start]
while True:
    _nexts=[]
    for n in nexts:
        for neigh in neighbor[n]:
            assert neigh!=-1
            if dist[neigh]>dist[n]+1:
                _nexts.append(neigh)
                dist[neigh]=dist[n]+1
    nexts=set(_nexts)
    if len(nexts)==0:
        break

if dist[goal]==10**10:
    print(-1)
else:
    print(dist[goal])

Buggy python submission (verdict: Runtime Error):
H,W=[int(i) for i in input().split(" ")]
Ch, Cw=[int(i)+1 for i in input().split(" ")]
Dh,Dw=[int(i)+1 for i in input().split(" ")]
S=[[False for i in range(W+4)],[False for i in range(W+4)]]
for i in range(H):
    S.append([False,False]+[j=='.' for j in input()]+[False,False])
S+=[[False for i in range(W+4)],[False for i in range(W+4)]]

def check(h,w,island_num):
    assert S[h][w]
    block[h][w]=island_num
    if S[h-1][w] and block[h-1][w]==-1:
        check(h-1,w,island_num)
    if S[h+1][w] and block[h+1][w]==-1:
        check(h+1,w,island_num)
    if S[h][w-1] and block[h][w-1]==-1:
        check(h,w-1,island_num)
    if S[h][w+1] and block[h][w+1]==-1:
        check(h,w+1,island_num)

block=[]
for i in range(H+4):
    block.append([-1 for j in range(W+4)])

island_num=0

for h in range(H+4):
    for w in range(W+4):
        if S[h][w] and block[h][w]==-1:
            check(h,w,island_num)
            island_num+=1


neighbor={}
for island in range(island_num):
    neighbor[island]=set()


for h in range(2,H+2):
    for w in range(2,W+2):
        if block[h][w]!=-1:
            for y in range(h-2,h+3):
                for x in range(w-2, w+3):
                    if block[y][x]!=-1 and block[h][w]!=block[y][x]:
                        neighbor[block[h][w]].add(block[y][x])


start=block[Ch][Cw]
goal=block[Dh][Dw]
assert start!=-1
assert goal!=-1



dist=[10**10 for i in range(island_num)]
dist[start]=0

nexts=[start]
while len(nexts)!=0:
    for n in nexts:
        _nexts=[]
        for neigh in neighbor[n]:
            assert neigh!=-1
            if dist[neigh]>dist[n]+1:
                _nexts.append(neigh)
                dist[neigh]=dist[n]+1
        nexts=set(_nexts)

if dist[goal]==10**10:
    print(-1)
    assert dist[goal]=-1
else:
    print(dist[goal])

Fixed submission:
H,W=[int(i) for i in input().split(" ")]
Ch, Cw=[int(i)+1 for i in input().split(" ")]
Dh,Dw=[int(i)+1 for i in input().split(" ")]
S=[[False for i in range(W+4)],[False for i in range(W+4)]]
for i in range(H):
    S.append([False,False]+[j=='.' for j in input()]+[False,False])
S+=[[False for i in range(W+4)],[False for i in range(W+4)]]

def check(h,w,island_num):
    assert S[h][w]
    block[h][w]=island_num
    if S[h-1][w] and block[h-1][w]==-1:
        check(h-1,w,island_num)
    if S[h+1][w] and block[h+1][w]==-1:
        check(h+1,w,island_num)
    if S[h][w-1] and block[h][w-1]==-1:
        check(h,w-1,island_num)
    if S[h][w+1] and block[h][w+1]==-1:
        check(h,w+1,island_num)

block=[]
for i in range(H+4):
    block.append([-1 for j in range(W+4)])

island_num=0

for h in range(H+4):
    for w in range(W+4):
        if S[h][w] and block[h][w]==-1:
            check(h,w,island_num)
            island_num+=1


neighbor={}
for island in range(island_num):
    neighbor[island]=set()


for h in range(2,H+2):
    for w in range(2,W+2):
        if block[h][w]!=-1:
            for y in range(h-2,h+3):
                for x in range(w-2, w+3):
                    if block[y][x]!=-1 and block[h][w]!=block[y][x]:
                        neighbor[block[h][w]].add(block[y][x])


start=block[Ch][Cw]
goal=block[Dh][Dw]
assert start!=-1
assert goal!=-1



dist=[10**10 for i in range(island_num)]
dist[start]=0

nexts=[start]
while True:
    _nexts=[]
    for n in nexts:
        for neigh in neighbor[n]:
            assert neigh!=-1
            if dist[neigh]>dist[n]+1:
                _nexts.append(neigh)
                dist[neigh]=dist[n]+1
    nexts=set(_nexts)
    if len(nexts)==0:
        break

if dist[goal]==10**10:
    print(-1)
else:
    print(dist[goal])