Buggy python submission (verdict: Runtime Error):
import math
import copy
import sys
import fractions
# import numpy as np
# import statistics
import decimal
import heapq
import collections
import itertools
import bisect

# from operator import mul

sys.setrecursionlimit(100001)


# input = sys.stdin.readline
# sys.setrecursionlimit(10**6)

# ===FUNCTION===

def getInputIntList():
    outputDataList = []
    inputData = input().split()
    outputDataList = [int(n) for n in inputData]

    return outputDataList


def getSomeInputInt(n):
    outputDataList = []
    for i in range(n):
        inputData = int(input())
        outputDataList.append(inputData)

    return outputDataList


def getSomeInputListInt(n):
    inputDataList = []
    outputDataList = []
    for i in range(n):
        inputData = input().split()
        inputDataList = [int(n) for n in inputData]
        outputDataList.append(inputDataList)

    return outputDataList


# ===CODE===

h, w = map(int, input().split())

table = [[1 for _ in range(w)] for _ in range(h)]
d_table = [[-1 for _ in range(w)] for _ in range(h)]

for i in range(h):
    s = input()
    tmp = list(s)
    for j, ts in enumerate(tmp):
        if ts == ".":
            table[i][j] = 0

ans = -1
search_table = [[-1, 0], [1, 0], [0, -1], [0, 1]]

q = collections.deque([])
q.append([0,0])
table[0][0]=1
d_table[0][0]=0

gy, gx = [h-1, w-1]

while len(q) > 0:
    qy, qx = q.popleft()

    for y, x in search_table:
        cy = qy + y
        cx = qx + x

        if cy < 0 or cy > h - 1 or cx < 0 or cx > w - 1:
            continue

        if table[cy][cx] == 0 and d_table[cy][cx] == -1:
            table[cy][cx] = 1
            d_table[cy][cx] = d_table[qy][qx] + 1
            q.append([cy, cx])

            if cy == gy and cx == gx:
                ans = d_table[cy][cx]

if ans == -1:
    print(-1)
    exit(0)

cnt = [-1 for _ in range(h*w+1)]

for i in range(h):
    for j in range(w):
        tmp = d_table[i][j]
        if tmp > -1
            cnt[tmp]+=1

res = 0
for c in cnt:
    res += max(0, c)
print(res)


Fixed submission:
import math
import copy
import sys
import fractions
# import numpy as np
# import statistics
import decimal
import heapq
import collections
import itertools
import bisect

# from operator import mul

sys.setrecursionlimit(100001)


# input = sys.stdin.readline
# sys.setrecursionlimit(10**6)

# ===FUNCTION===

def getInputIntList():
    outputDataList = []
    inputData = input().split()
    outputDataList = [int(n) for n in inputData]

    return outputDataList


def getSomeInputInt(n):
    outputDataList = []
    for i in range(n):
        inputData = int(input())
        outputDataList.append(inputData)

    return outputDataList


def getSomeInputListInt(n):
    inputDataList = []
    outputDataList = []
    for i in range(n):
        inputData = input().split()
        inputDataList = [int(n) for n in inputData]
        outputDataList.append(inputDataList)

    return outputDataList


# ===CODE===

h, w = map(int, input().split())

table = [[1 for _ in range(w)] for _ in range(h)]
d_table = [[-1 for _ in range(w)] for _ in range(h)]

for i in range(h):
    s = input()
    tmp = list(s)
    for j, ts in enumerate(tmp):
        if ts == ".":
            table[i][j] = 0

ans = -1
search_table = [[-1, 0], [1, 0], [0, -1], [0, 1]]

q = collections.deque([])
q.append([0,0])
table[0][0]=1
d_table[0][0]=0

gy, gx = [h-1, w-1]

while len(q) > 0:
    qy, qx = q.popleft()

    for y, x in search_table:
        cy = qy + y
        cx = qx + x

        if cy < 0 or cy > h - 1 or cx < 0 or cx > w - 1:
            continue

        if table[cy][cx] == 0 and d_table[cy][cx] == -1:
            table[cy][cx] = 1
            d_table[cy][cx] = d_table[qy][qx] + 1
            q.append([cy, cx])

            if cy == gy and cx == gx:
                ans = d_table[cy][cx]

if ans == -1:
    print(-1)
    exit(0)

cnt = 0

for i in range(h):
    for j in range(w):
        tmp = d_table[i][j]
        if tmp > -1:
            cnt+=1
        elif tmp == -1 and table[i][j]==0:
            cnt+=1

res = cnt -ans -1



print(res)