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 Rust (1.15.1) (judge verdict: Accepted)

use std::cmp::Ordering;
use std::collections::VecDeque;
use std::fmt::Debug;
#[allow(unused_imports)]
use std::fs::File;
#[allow(unused_imports)]
use std::io::{stdin, stdout, BufReader, BufWriter, Read, Write};
use std::str::FromStr;

pub fn start<R: Read, W: Write>(input: R, output: &mut W) {
    let mut scanner = Scanner::new(input.bytes().map(Result::unwrap));
    let h = scanner.next_value::<usize>();
    let w = scanner.next_value::<usize>();
    let c = scanner.next_2d_bytes(h);

    let mut q = VecDeque::new();

    let s: usize = c
        .iter()
        .map(|r| r.iter().filter(|&&ce| ce == b'.').count())
        .sum();

    q.push_back((0, 0, 1));
    let mut t = vec![vec![u32::max_value(); w]; h];
    while !q.is_empty() {
        let pos = q.pop_front().unwrap();
        if t[pos.0][pos.1] <= pos.2 {
            continue;
        }
        if c[pos.0][pos.1] == b'#' {
            continue;
        }
        if pos.0 == h - 1 && pos.1 == w - 1 {
            writeln!(output, "{}", s as u32 - pos.2).unwrap();
            return;
        }
        t[pos.0][pos.1] = pos.2;
        if pos.0 > 0 {
            q.push_back((pos.0 - 1, pos.1, pos.2 + 1));
        }
        if pos.0 < h - 1 {
            q.push_back((pos.0 + 1, pos.1, pos.2 + 1));
        }
        if pos.1 > 0 {
            q.push_back((pos.0, pos.1 - 1, pos.2 + 1));
        }
        if pos.1 < w - 1 {
            q.push_back((pos.0, pos.1 + 1, pos.2 + 1));
        }
        // for r in t.iter() {
        //     for &c in r.iter() {
        //         if c == u16::max_value() {
        //             print!("|__");
        //         } else {
        //             print!("|{: >2}", c);
        //         }
        //     }
        // println!("|");
        // }
        // println!("");
    }
    writeln!(output, "-1").unwrap();
}

////////////////////////////
// Tests
////////////////////////////

#[cfg(test)]
mod test {
    use super::start;
    use std::ffi::OsStr;
    use std::fs;
    use std::fs::File;
    use std::io::Cursor;
    use std::io::{BufReader, Read};

    #[test]
    fn example() {
        let mut entries = fs::read_dir("./tests")
            .unwrap()
            .into_iter()
            .flat_map(|x| x.ok())
            .map(|x| x.path())
            .filter(|x| x.extension().unwrap_or(OsStr::new("")).to_string_lossy() == "in")
            .collect::<Vec<_>>();
        entries.sort();
        for entry in entries {
            let mut output = Vec::new();
            BufReader::new(File::open(entry.with_extension("out")).unwrap())
                .read_to_end(&mut output)
                .unwrap();
            let input = BufReader::new(File::open(entry).unwrap());

            let mut out = Cursor::new(Vec::new());
            start(input, &mut out);
            let outvec = out.into_inner();
            println!("{}", String::from_utf8((&outvec).to_vec()).unwrap());
            assert_eq!(outvec, output);
        }
    }
}

////////////////////////////
// Snipetts
////////////////////////////

#[allow(dead_code)]
fn main() {
    let stdin = stdin();
    let stdout = stdout();
    let mut out = BufWriter::new(stdout.lock());
    let input = BufReader::new(stdin.lock());
    start(input, &mut out);
}

// // ファイルの場合
// #[allow(dead_code)]
// fn main() {
//     let mut out = BufWriter::new(File::create("output.txt").unwrap());
//     let input = BufReader::new(File::open("input.txt").unwrap());
//     start(input, &mut out);
// }

// 大きなスタックが必要な場合はこちらを利用する
// #[allow(dead_code)]
// fn main() {
//     std::thread::Builder::new()
//         .name("big stack size".into())
//         .stack_size(32 * 1024 * 1024) // 32 MBのスタックサイズ
//         .spawn(|| {
//             let stdin = stdin();
//             let stdout = stdout();
//             let out = BufWriter::new(stdout.lock());
//             let input = BufReader::new(stdin.lock());
//             start(input, out);
//         })
//         .unwrap()
//         .join()
//         .unwrap();
// }

pub struct Scanner<I: Iterator<Item = u8>> {
    input: I,
}

impl<I: Iterator<Item = u8>> Scanner<I> {
    pub fn new(input: I) -> Self {
        Scanner::<I> { input: input }
    }

    // 文字コードはISO/IEC 8859-1 (Latin-1)とする
    pub fn next_value<T: FromStr>(&mut self) -> T
    where
        <T as FromStr>::Err: Debug,
    {
        T::from_str(
            &(self
                .input
                .by_ref()
                .map(char::from)
                .skip_while(|x| x.is_whitespace())
                .take_while(|x| !x.is_whitespace())
                .collect::<String>()),
        )
        .unwrap()
    }

    // 文字コードはISO/IEC 8859-1 (Latin-1)とする
    #[allow(dead_code)]
    pub fn next_bytes(&mut self) -> Vec<u8> {
        self.input
            .by_ref()
            .skip_while(|&x| char::from(x).is_whitespace())
            .take_while(|&x| !char::from(x).is_whitespace())
            .collect::<Vec<_>>()
    }

    #[allow(dead_code)]
    pub fn next_array<T: FromStr>(&mut self, len: usize) -> Vec<T>
    where
        <T as FromStr>::Err: Debug,
    {
        (0..len).map(|_| self.next_value()).collect()
    }

    #[allow(dead_code)]
    pub fn next_2darray<T: FromStr>(&mut self, x: usize, y: usize) -> Vec<Vec<T>>
    where
        <T as FromStr>::Err: Debug,
    {
        (0..y).map(|_| self.next_array(x)).collect()
    }

    #[allow(dead_code)]
    pub fn next_2d_bytes(&mut self, len: usize) -> Vec<Vec<u8>> {
        (0..len).map(|_| self.next_bytes()).collect()
    }
}

// 1.15.1でReverseを使う
// https://qiita.com/hatoo@github/items/fa14ad36a1b568d14f3e
#[derive(Eq, PartialEq, Clone, Debug)]
pub struct Reverse<T>(pub T);

impl<T: PartialOrd> PartialOrd for Reverse<T> {
    fn partial_cmp(&self, other: &Reverse<T>) -> Option<Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl<T: Ord> Ord for Reverse<T> {
    fn cmp(&self, other: &Reverse<T>) -> Ordering {
        other.0.cmp(&self.0)
    }
}

// PartialOrdをTotalOrdにするラッパ
// https://qiita.com/hatoo@github/items/fa14ad36a1b568d14f3e

#[derive(PartialEq, PartialOrd)]
pub struct Total<T>(pub T);

impl<T: PartialEq> Eq for Total<T> {}

impl<T: PartialOrd> Ord for Total<T> {
    fn cmp(&self, other: &Total<T>) -> Ordering {
        self.0.partial_cmp(&other.0).unwrap()
    }
}

// 順列生成
#[allow(dead_code)]
pub fn permutations(n: usize, k: usize) -> Box<Iterator<Item = Vec<u8>>> {
    fn inner(array: Vec<u8>, depth: usize, max: u8) -> Box<Iterator<Item = Vec<u8>>> {
        let na = (0..max).flat_map(move |x| {
            if array.contains(&x) {
                None
            } else {
                let mut newarray = array.clone();
                newarray.push(x);
                Some(newarray)
            }
        });
        if depth > 0 {
            Box::new(na.flat_map(move |x| inner(x, depth - 1, max)))
        } else {
            Box::new(na)
        }
    }
    inner(Vec::new(), k - 1, n as u8)
}

// 組み合わせ生成
#[allow(dead_code)]
pub fn combinations(n: usize, r: usize) -> Box<Iterator<Item = Vec<u8>>> {
    fn inner(
        array: Vec<u8>,
        depth: usize,
        min: usize,
        max: usize,
    ) -> Box<Iterator<Item = Vec<u8>>> {
        let na = (min..(max - depth)).map(move |x| {
            let mut newarray = array.clone();
            newarray.push(x as u8);
            newarray
        });
        if depth > 0 {
            Box::new(na.flat_map(move |x| {
                let next = usize::from(*x.last().unwrap()) + 1;
                inner(x, depth - 1, next, max)
            }))
        } else {
            Box::new(na)
        }
    }
    inner(Vec::new(), r - 1, 0, n)
}