Buggy python submission (verdict: Runtime Error):
H, W = map(int, input().split())
s=[]
for row in range(H):
    s.append(input())
#print(s)



waiting=[]
dx=[1,0,-1,0]
dy=[0,1,0,-1]
def func(xy):
    x, y = xy
    for i in range(4):
        if 0<=y+dy[i]<H and 0<=x+dx[i]<W:
            if walker[y+dy[i]][x+dx[i]] == None and s[y+dy[i]][x+dx[i]]=='.':
                walker[y + dy[i]][x + dx[i]]=walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0))
    else:
        return None

walker=[]
for row in range(H):
    walker.append([None]*W)
#print(walker)
walker[0][0]=0
func((0,0))
#print(walker)
if walker[H-1][W-1] != None:
    minimum=walker[H-1][W-1]

    white_count=0
    for row in range(H):
        white_count += s[row].count('.')
    #print(white_count)
    print(white_count-minimum-1)
else:
    print(-1)



Fixed submission:
import sys
sys.setrecursionlimit(5000)

H, W = map(int, input().split())
s=[]
for row in range(H):
    s.append(input())
#print(s)



waiting=[]
dx=[1,0,-1,0]
dy=[0,1,0,-1]
def func(xy):
    x, y = xy
    for i in range(4):
        if 0<=y+dy[i]<H and 0<=x+dx[i]<W:
            if walker[y+dy[i]][x+dx[i]] == None and s[y+dy[i]][x+dx[i]]=='.':
                walker[y + dy[i]][x + dx[i]]=walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0))
    else:
        return None

walker=[]
for row in range(H):
    walker.append([None]*W)
#print(walker)
walker[0][0]=0
func((0,0))
#print(walker)
if walker[H-1][W-1] != None:
    minimum=walker[H-1][W-1]

    white_count=0
    for row in range(H):
        white_count += s[row].count('.')
    #print(white_count)
    print(white_count-minimum-1)
else:
    print(-1)

Buggy python submission (verdict: Runtime Error):
H, W = map(int, input().split())
s=[]
for row in range(H):
    s.append(input())
#print(s)



waiting=[]
dx=[1,0,-1,0]
dy=[0,1,0,-1]
def func(xy):
    x, y = xy
    for i in range(4):
        if 0<=y+dy[i]<H and 0<=x+dx[i]<W:
            if walker[y+dy[i]][x+dx[i]] == None and s[y+dy[i]][x+dx[i]]=='.':
                walker[y + dy[i]][x + dx[i]]=walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0))
    else:
        return None

walker=[]
for row in range(H):
    walker.append([None]*W)
#print(walker)
walker[0][0]=0
func((0,0))
#print(walker)
minimum=walker[H-1][W-1]

white_count=0
for row in range(H):
    white_count += s[row].count('.')
#print(white_count)
print(white_count-minimum-1)



Fixed submission:
import sys
sys.setrecursionlimit(5000)

H, W = map(int, input().split())
s=[]
for row in range(H):
    s.append(input())
#print(s)



waiting=[]
dx=[1,0,-1,0]
dy=[0,1,0,-1]
def func(xy):
    x, y = xy
    for i in range(4):
        if 0<=y+dy[i]<H and 0<=x+dx[i]<W:
            if walker[y+dy[i]][x+dx[i]] == None and s[y+dy[i]][x+dx[i]]=='.':
                walker[y + dy[i]][x + dx[i]]=walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0))
    else:
        return None

walker=[]
for row in range(H):
    walker.append([None]*W)
#print(walker)
walker[0][0]=0
func((0,0))
#print(walker)
if walker[H-1][W-1] != None:
    minimum=walker[H-1][W-1]

    white_count=0
    for row in range(H):
        white_count += s[row].count('.')
    #print(white_count)
    print(white_count-minimum-1)
else:
    print(-1)