Problem:
Score: 400 points

Problem statement

We have an H \times W grid whose squares are painted black or white. The square at the i-th row from the top and the j-th column from the left is denoted as (i, j).

Snuke would like to play the following game on this grid. At the beginning of the game, there is a character called Kenus at square (1, 1). The player repeatedly moves Kenus up, down, left or right by one square. The game is completed when Kenus reaches square (H, W) passing only white squares.

Before Snuke starts the game, he can change the color of some of the white squares to black. However, he cannot change the color of square (1, 1) and (H, W). Also, changes of color must all be carried out before the beginning of the game.

When the game is completed, Snuke's score will be the number of times he changed the color of a square before the beginning of the game. Find the maximum possible score that Snuke can achieve, or print -1 if the game cannot be completed, that is, Kenus can never reach square (H, W) regardless of how Snuke changes the color of the squares.

The color of the squares are given to you as characters s_{i, j}. If square (i, j) is initially painted by white, s_{i, j} is .; if square (i, j) is initially painted by black, s_{i, j} is #.

Constraints

H is an integer between 2 and 50 (inclusive).

W is an integer between 2 and 50 (inclusive).

s_{i, j} is . or # (1 \leq i \leq H, 1 \leq j \leq W).

s_{1, 1} and s_{H, W} are ..

Input

Input is given from Standard Input in the following format:

H W
s_{1, 1}s_{1, 2}s_{1, 3} ... s_{1, W}
s_{2, 1}s_{2, 2}s_{2, 3} ... s_{2, W}
: :
s_{H, 1}s_{H, 2}s_{H, 3} ... s_{H, W}

Output

Print the maximum possible score that Snuke can achieve, or print -1 if the game cannot be completed.

Sample Input 1

3 3
..#
#..
...

Sample Output 1

2

The score 2 can be achieved by changing the color of squares as follows:

Sample Input 2

10 37
.....................................
...#...####...####..###...###...###..
..#.#..#...#.##....#...#.#...#.#...#.
..#.#..#...#.#.....#...#.#...#.#...#.
.#...#.#..##.#.....#...#.#.###.#.###.
.#####.####..#.....#...#..##....##...
.#...#.#...#.#.....#...#.#...#.#...#.
.#...#.#...#.##....#...#.#...#.#...#.
.#...#.####...####..###...###...###..
.....................................

Sample Output 2

209

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        public static int[,] map = null;
        public static int[,] marker = null;
        public static int N = 0;
        public static int W = 0;
        public static int H = 0;
        public static bool ret = false;
        public static int count = 0;

        public static List<Point> que = new List<Point>();
        // チーズの硬さと座標
        public static Dictionary<int, Point> goal_list = new Dictionary<int, Point>();
        public static List<int> goal_root = new List<int>();

        static void Main(string[] args)
        {
            var data = new List<int>();

            count = 0;

            var sx = 0;
            var sy = 0;
            var s = Console.ReadLine().Split(' ');
            H = int.Parse(s[0]);
            W = int.Parse(s[1]);

            map = new int[W, H];
            marker = new int[W, H];

            // マップ読み込み
            for (int y = 0; y < H; y++)
            {
                var tmp = Console.ReadLine();
                for (int x = 0; x < tmp.Length; x++)
                {
                    map[x, y] = ('#' == tmp[x]) ? 0 : 1;
                    // 空白マスをカウント
                    count += ('.' == tmp[x]) ? 1 : 0;
                }
            }

            var ex = W - 1;
            var ey = H - 1;

            que.Add(new Point(sx, sy));

            // -1で初期化                
            for (int y = 0; y < H; y++)
                for (int x = 0; x < W; x++)
                    marker[x, y] = -1;

            marker[sx, sy] = 0;

            var ret = false;
            while (0 < que.Count)
            {
                var x = que[0].X;
                var y = que[0].Y;

                // ゴールに到達していたら終了
                if (x == ex && y == ey)
                {
                    ret = true;
                    break;
                }

                AddQue(x, y);
                que.RemoveAt(0);
            }

            // 空白マス数から最短経路の距離を引いたマスだけ塗り潰せる(始点をマイナス1)
            count = count - marker[ex, ey] - 1;
            // 塗り潰せない場合は0を出力
            if (count <= 0) count = 0;
            // そもそもゴールに到達していなかったら-1
            if (!ret) count = -1;
            Console.WriteLine(count);
            Console.Read();
        }

        public static void AddQue(int x, int y)
        {
            var distance = marker[x, y];
            Search(x + 1, y, distance + 1);
            Search(x - 1, y, distance + 1);
            Search(x, y + 1, distance + 1);
            Search(x, y - 1, distance + 1);
        }

        public static bool Search(int x, int y, int distance)
        {
            if (x < 0 || y < 0 || W <= x || H <= y || -1 != marker[x, y] || 1 != map[x, y]) return false;

            marker[x, y] = distance;
            que.Add(new Point(x, y));

            return true;
        }
    }

    public class Point
    {
        public int X { get; set; } = 0;
        public int Y { get; set; } = 0;
        public Point() { }
        public Point(int x, int y) { X = x; Y = y; }
    }
}