Buggy python submission (verdict: Time Limit Exceeded):
import sys;input=sys.stdin.readline
H, W = map(int, input().split())
cx, cy = map(int, input().split())
dx, dy = map(int, input().split())
cx-=1;cy-=1;dx-=1;dy-=1
B = []
for _ in range(H):
    B.append(input().strip())

D = [0]*(H*W)
bb = -1
t = 0
t2c = dict()
for i in range(H):
    for j in range(W):
        if B[i][j] == "#":
            D[i*W+j] = bb
            continue
        if D[i*W+j]>0:
            continue
        t += 1
        t2c[t] = set()
        stack = [(i, j)]
        while stack:
            u, v = stack.pop()
            D[W*u+v] = t
            t2c[t].add((u, v))
            for x, y in [(u+1, v), (u-1, v), (u, v-1), (u, v+1)]:
                if x >= H or x<0:
                    continue
                if y >= W or y<0:
                    continue
                if B[x][y] == "#":
                    D[x*W+y] = bb
                    continue
                if D[x*W+y]:
                    continue
                stack.append((x, y))
depth = 0
cs = [D[cx*W+cy]]
tt = D[dx*W+dy]
vs = set([cs[0], -1])
if tt in vs:
    print(0)
    sys.exit()
while True:
    depth += 1
    nexts = []
    for c in cs:
        for i, j in t2c[c]:
            for ii in range(max(0, i-2), min(i+3, H)):
                for jj in range(max(0, j-2), min(j+3, W)):
                    if D[ii*W+jj] in vs:
                        continue
                    nexts.append(D[ii*W+jj])
                    if D[ii*W+jj] == tt:
                        print(depth)
                        sys.exit()
    cs = nexts
    if not cs:
        break
print(-1)


Fixed submission:
import sys;input=sys.stdin.readline
H, W = map(int, input().split())
cx, cy = map(int, input().split())
dx, dy = map(int, input().split())
cx-=1;cy-=1;dx-=1;dy-=1
B = []
for _ in range(H):
    B.append(input().strip())

D = [0]*(H*W)
bb = -1
t = 0
t2c = dict()
for i in range(H):
    for j in range(W):
        if B[i][j] == "#":
            D[i*W+j] = bb
            continue
        if D[i*W+j]>0:
            continue
        t += 1
        t2c[t] = set()
        stack = [(i, j)]
        while stack:
            u, v = stack.pop()
            D[W*u+v] = t
            t2c[t].add((u, v))
            for x, y in [(u+1, v), (u-1, v), (u, v-1), (u, v+1)]:
                if x >= H or x<0:
                    continue
                if y >= W or y<0:
                    continue
                if B[x][y] == "#":
                    D[x*W+y] = bb
                    continue
                if D[x*W+y]:
                    continue
                stack.append((x, y))
depth = 0
cs = [D[cx*W+cy]]
tt = D[dx*W+dy]
vs = set([cs[0], -1])
if tt in vs:
    print(0)
    sys.exit()
while True:
    depth += 1
    nexts = []
    for c in cs:
        for i, j in t2c[c]:
            for ii in range(max(0, i-2), min(i+3, H)):
                for jj in range(max(0, j-2), min(j+3, W)):
                    if D[ii*W+jj] in vs:
                        continue
                    vs.add(D[ii*W+jj])
                    nexts.append(D[ii*W+jj])
                    if D[ii*W+jj] == tt:
                        print(depth)
                        sys.exit()
    cs = nexts
    if not cs:
        break
print(-1)

Buggy python submission (verdict: Time Limit Exceeded):
import sys;input=sys.stdin.readline
H, W = map(int, input().split())
cx, cy = map(int, input().split())
dx, dy = map(int, input().split())
cx-=1;cy-=1;dx-=1;dy-=1
B = []
for _ in range(H):
    B.append(input().strip())

D = [0]*(H*W)
bb = -1
t = 0
t2c = dict()
for i in range(H):
    for j in range(W):
        if B[i][j] == "#":
            D[i*W+j] = bb
            continue
        if D[i*W+j]>0:
            continue
        t += 1
        t2c[t] = set()
        stack = [(i, j)]
        while stack:
            u, v = stack.pop()
            D[W*u+v] = t
            t2c[t].add((u, v))
            for x, y in [(u+1, v), (u-1, v), (u, v-1), (u, v+1)]:
                if x >= H or x<0:
                    continue
                if y >= W or y<0:
                    continue
                if B[x][y] == "#":
                    D[x*W+y] = bb
                    continue
                if D[x*W+y]:
                    continue
                stack.append((x, y))
depth = 0
cs = [D[cx*W+cy]]
tt = D[dx*W+dy]
vs = set([cs[0]])
if tt in vs:
    print(0)
    sys.exit()
while True:
    depth += 1
    nexts = []
    for c in cs:
        for i, j in t2c[c]:
            for ii in range(max(0, i-2), min(i+3, H)):
                for jj in range(max(0, j-2), min(j+3, W)):
                    if B[ii][jj] == "#":
                        continue
                    if D[ii*W+jj] in vs:
                        continue
                    nexts.append(D[ii*W+jj])
                    if D[ii*W+jj] == tt:
                        print(depth)
                        sys.exit()
    cs = nexts
    if not cs:
        break
print(-1)


Fixed submission:
import sys;input=sys.stdin.readline
H, W = map(int, input().split())
cx, cy = map(int, input().split())
dx, dy = map(int, input().split())
cx-=1;cy-=1;dx-=1;dy-=1
B = []
for _ in range(H):
    B.append(input().strip())

D = [0]*(H*W)
bb = -1
t = 0
t2c = dict()
for i in range(H):
    for j in range(W):
        if B[i][j] == "#":
            D[i*W+j] = bb
            continue
        if D[i*W+j]>0:
            continue
        t += 1
        t2c[t] = set()
        stack = [(i, j)]
        while stack:
            u, v = stack.pop()
            D[W*u+v] = t
            t2c[t].add((u, v))
            for x, y in [(u+1, v), (u-1, v), (u, v-1), (u, v+1)]:
                if x >= H or x<0:
                    continue
                if y >= W or y<0:
                    continue
                if B[x][y] == "#":
                    D[x*W+y] = bb
                    continue
                if D[x*W+y]:
                    continue
                stack.append((x, y))
depth = 0
cs = [D[cx*W+cy]]
tt = D[dx*W+dy]
vs = set([cs[0], -1])
if tt in vs:
    print(0)
    sys.exit()
while True:
    depth += 1
    nexts = []
    for c in cs:
        for i, j in t2c[c]:
            for ii in range(max(0, i-2), min(i+3, H)):
                for jj in range(max(0, j-2), min(j+3, W)):
                    if D[ii*W+jj] in vs:
                        continue
                    vs.add(D[ii*W+jj])
                    nexts.append(D[ii*W+jj])
                    if D[ii*W+jj] == tt:
                        print(depth)
                        sys.exit()
    cs = nexts
    if not cs:
        break
print(-1)