Buggy python submission (verdict: Runtime Error):
from collections import deque

def bfs(board, visited, gh, gw):
    turn = 0
    queue = deque([[0, 0, turn]])
    while queue:
        h, w, turn = queue.popleft()
        if h == gh-1 and w == gw-1:
            return turn
        for i, j in ([1,0],[-1,0],[0,1],[0,-1]):
            new_h, new_w = h+i, w+j
            if new_h >= gh or new_h < 0 or new_w >= gw or new_w < 0:
                continue
            elif board[new_h][new_w] != "#" and visited[new_h][new_w] == "x":
                visited[new_h][new_w] = "o"
                queue.append([new_h, new_w, turn+1])

def main():
    H, W = map(int,input().split())
    board = []
    for _ in range(H):
        board.append(list(input()))
    black = 0
    for i in range(H):
        for j in range(W):
            if board[i][j] == "#":
                black += 1
    visited = [["x"]*W for _ in range(H)]
    path_area = bfs(board, visited, H, W) + 1
    print(H*W-path_area-black)
                
    
    
if __name__ == "__main__":
    main()

Fixed submission:
from collections import deque

def bfs(board, visited, gh, gw):
    turn = 0
    queue = deque([[0, 0, turn]])
    while queue:
        h, w, turn = queue.popleft()
        if h == gh-1 and w == gw-1:
            return turn
        for i, j in ([1,0],[-1,0],[0,1],[0,-1]):
            new_h, new_w = h+i, w+j
            if new_h >= gh or new_h < 0 or new_w >= gw or new_w < 0:
                continue
            elif board[new_h][new_w] != "#" and visited[new_h][new_w] == "x":
                visited[new_h][new_w] = "o"
                queue.append([new_h, new_w, turn+1])
    return -1

def main():
    H, W = map(int,input().split())
    board = []
    for _ in range(H):
        board.append(list(input()))
    black = 0
    for i in range(H):
        for j in range(W):
            if board[i][j] == "#":
                black += 1
    visited = [["x"]*W for _ in range(H)]
    path_area = bfs(board, visited, H, W) + 1
    if path_area:
        print(H*W-path_area-black)
    else:
        print(-1)
                
    
    
if __name__ == "__main__":
    main()

Buggy python submission (verdict: Runtime Error):
from collections import deque

def bfs(board, visited, gh, gw):
    turn = 0
    queue = deque([[0, 0, turn]])
    while queue:
        h, w, turn = queue.popleft()
        if h == gh-1 and w == gw-1:
            return turn
        for i, j in ([1,0],[-1,0],[0,1],[0,-1]):
            new_h, new_w = h+i, w+j
            if new_h >= gh or new_h < 0 or new_w >= gw or new_w < 0:
                continue
            elif board[new_h][new_w] != "#" and visited[new_h][new_w] == "x":
                visited[new_h][new_w] = "o"
                queue.append([new_h, new_w, turn+1])
    
    

def main():
    H, W = map(int,input().split())
    board = []
    for _ in range(H):
        board.append(list(input()))
    black = 0
    for i in range(H):
        for j in range(W):
            if board[i][j] == "#":
                black += 1
    visited = [["x"]*W for _ in range(H)]
    path_area = bfs(board, visited, H, W) + 1
    print(H*W-path_area-black)
                
    
    
if __name__ == "__main__":
    main()

Fixed submission:
from collections import deque

def bfs(board, visited, gh, gw):
    turn = 0
    queue = deque([[0, 0, turn]])
    while queue:
        h, w, turn = queue.popleft()
        if h == gh-1 and w == gw-1:
            return turn
        for i, j in ([1,0],[-1,0],[0,1],[0,-1]):
            new_h, new_w = h+i, w+j
            if new_h >= gh or new_h < 0 or new_w >= gw or new_w < 0:
                continue
            elif board[new_h][new_w] != "#" and visited[new_h][new_w] == "x":
                visited[new_h][new_w] = "o"
                queue.append([new_h, new_w, turn+1])
    return -1

def main():
    H, W = map(int,input().split())
    board = []
    for _ in range(H):
        board.append(list(input()))
    black = 0
    for i in range(H):
        for j in range(W):
            if board[i][j] == "#":
                black += 1
    visited = [["x"]*W for _ in range(H)]
    path_area = bfs(board, visited, H, W) + 1
    if path_area:
        print(H*W-path_area-black)
    else:
        print(-1)
                
    
    
if __name__ == "__main__":
    main()