Buggy python submission (verdict: Runtime Error):
from collections import deque
def bfs(field,visited,sy,sx,gy,gx):
    queue = deque([[sy,sx]])
    visited[sy][sx] = 0
    while queue:
        y,x = queue.popleft()
        if [y,x] == [gy,gx]:
            return visited[y][x]

        for j,k in ([1,0],[-1,0],[0,1],[0,-1]):
            new_y, new_x = y+j, x+k
            if new_y < 0 or new_y > gy or new_x < 0 or new_x > gx:
              continue
            if field[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])

if __name__ == "__main__":

    h,w=map(int,input().split())
    sy=1
    sx=1
    field=[list(input()) for i in range(h)]
    visited = [[-1]*w for i in range(h)]
    sy, sx, gy, gx = sy-1, sx-1, h-1, w-1
    ans = bfs(field,visited,sy,sx,gy,gx)
    cnt=0
    for i in range(h):
      for j in range(w):
        if field[i][j]==".":
          cnt+=1
    if ans==None:
      print(-1)
    else:
      print(cnt - bfs(field,visited,sy,sx,gy,gx) -1)

Fixed submission:
from collections import deque
def bfs(field,visited,sy,sx,gy,gx):
    queue = deque([[sy,sx]])
    visited[sy][sx] = 0
    while queue:
        y,x = queue.popleft()
        if [y,x] == [gy,gx]:
            return visited[y][x]
 
        for j,k in ([1,0],[-1,0],[0,1],[0,-1]):
            new_y, new_x = y+j, x+k
            if new_y < 0 or new_y > gy or new_x < 0 or new_x > gx:
              continue
            if field[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])
                
if __name__ == "__main__":
 
    h,w=map(int,input().split())
    sy=1
    sx=1
    field=[list(input()) for i in range(h)]
    visited = [[-1]*w for i in range(h)]
    sy, sx, gy, gx = sy-1, sx-1, h-1, w-1
    ans=bfs(field,visited,sy,sx,gy,gx)
    #print(ans)
    cnt=0
    for i in range(h):
      for j in range(w):
        if field[i][j]==".":
          cnt+=1
    if ans == None:
      print(-1)
    else:
      print(cnt - ans -1)

Buggy python submission (verdict: Runtime Error):
from collections import deque
def bfs(field,visited,sy,sx,gy,gx):
    queue = deque([[sy,sx]])
    visited[sy][sx] = 0
    while queue:
        y,x = queue.popleft()
        if [y,x] == [gy,gx]:
            return visited[y][x]

        for j,k in ([1,0],[-1,0],[0,1],[0,-1]):
            new_y, new_x = y+j, x+k
            if new_y < 0 or new_y > gy or new_x < 0 or new_x > gx:
              continue
            if field[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])

if __name__ == "__main__":

    h,w=map(int,input().split())
    sy=1
    sx=1
    field=[list(input()) for i in range(h)]
    visited = [[-1]*w for i in range(h)]
    sy, sx, gy, gx = sy-1, sx-1, h-1, w-1
    #print(bfs(field,visited,sy,sx,gy,gx))
    cnt=0
    for i in range(h):
      for j in range(w):
        if field[i][j]==".":
          cnt+=1
          
    print(cnt - bfs(field,visited,sy,sx,gy,gx) -1)

Fixed submission:
from collections import deque
def bfs(field,visited,sy,sx,gy,gx):
    queue = deque([[sy,sx]])
    visited[sy][sx] = 0
    while queue:
        y,x = queue.popleft()
        if [y,x] == [gy,gx]:
            return visited[y][x]
 
        for j,k in ([1,0],[-1,0],[0,1],[0,-1]):
            new_y, new_x = y+j, x+k
            if new_y < 0 or new_y > gy or new_x < 0 or new_x > gx:
              continue
            if field[new_y][new_x] == "." and visited[new_y][new_x] == -1:
                visited[new_y][new_x] = visited[y][x] + 1
                queue.append([new_y, new_x])
                
if __name__ == "__main__":
 
    h,w=map(int,input().split())
    sy=1
    sx=1
    field=[list(input()) for i in range(h)]
    visited = [[-1]*w for i in range(h)]
    sy, sx, gy, gx = sy-1, sx-1, h-1, w-1
    ans=bfs(field,visited,sy,sx,gy,gx)
    #print(ans)
    cnt=0
    for i in range(h):
      for j in range(w):
        if field[i][j]==".":
          cnt+=1
    if ans == None:
      print(-1)
    else:
      print(cnt - ans -1)