Problem:
Score : 400 points

Problem Statement

Consider a grid with H rows and W columns of squares. Let (r, c) denote the square at the r-th row from the top and the c-th column from the left.
Each square is painted black or white.

The grid is said to be good if and only if the following condition is satisfied:

From (1, 1), we can reach (H, W) by moving one square right or down repeatedly, while always being on a white square.

Note that (1, 1) and (H, W) must be white if the grid is good.

Your task is to make the grid good by repeating the operation below. Find the minimum number of operations needed to complete the task. It can be proved that you can always complete the task in a finite number of operations.

Choose four integers r_0, c_0, r_1, c_1(1 \leq r_0 \leq r_1 \leq H, 1 \leq c_0 \leq c_1 \leq W). For each pair r, c (r_0 \leq r \leq r_1, c_0 \leq c \leq c_1), invert the color of (r, c) - that is, from white to black and vice versa.

Constraints

2 \leq H, W \leq 100

Input

Input is given from Standard Input in the following format:

H W
s_{11} s_{12} \cdots s_{1W}
s_{21} s_{22} \cdots s_{2W}
\vdots
s_{H1} s_{H2} \cdots s_{HW}

Here s_{rc} represents the color of (r, c) - # stands for black, and . stands for white.

Output

Print the minimum number of operations needed.

Sample Input 1

3 3
.##
.#.
##.

Sample Output 1

1

Do the operation with (r_0, c_0, r_1, c_1) = (2, 2, 2, 2) to change just the color of (2, 2), and we are done.

Sample Input 2

2 2
#.
.#

Sample Output 2

2

Sample Input 3

4 4
..##
#...
###.
###.

Sample Output 3

0

No operation may be needed.

Sample Input 4

5 5
.#.#.
#.#.#
.#.#.
#.#.#
.#.#.

Sample Output 4

4

Solution in PHP7 (7.0.15) (judge verdict: Wrong Answer)

<?php

    define('WHITE', '.');
    define('BLACK', '#');

    // 記念提出

    list($h, $w) = in_ints();
    $cells = [];
    while ($in = in()) {
        $cells[] = str_split($in);
    }
    $cls = new RangeFlipFindRoute($cells);
    $cnt = $cls->solve();
    out($cnt);

    class RangeFlipFindRoute
    {
        private $cells;
        private $leaves_start;

        function __construct($cells) {
            $this->cells = $cells;
        }

        public function solve() {
            $this->debug();
            var_dump($this->is_good());
            return 0;
        }

        private function debug() {
            foreach ($this->cells as $c) {
                out(implode('', $c));
            }
        }

        private function flip_color($r0, $c0, $r1, $c1) {
            for ($i = $r0; $i <= $r1; $i++) {
                for ($j = $c0; $j <= $c1; $j++) {
                    $this->cells[$i][$j] = get_another_color($this->cells[$i][$j]);
                }
            }
        }

        private function is_good() {
            if ($this->cells[0][0] == BLACK) { // 始点が黒
                return false;
            }
            $row_cnt = count($this->cells);
            $col_cnt = count($this->cells[0]);
            if ($this->cells[$row_cnt - 1][$col_cnt - 1] == BLACK) { // 終点が黒
                return false;
            }
            $this->leaves = [];
            return $this->find_route(0, 0);
        }

        private function find_route($i, $j) {
            $row_goal = count($this->cells) - 1;
            $col_goal = count($this->cells[0]) - 1;
            $next = [];
            if ($i + 1 <= $row_goal && $this->cells[$i + 1][$j] == WHITE) {
                if ($i + 1 == $row_goal && $j == $col_goal) {
                    return true;
                }
                $next[] = [$i + 1, $j];
            }
            if ($j + 1 <= $col_goal && $this->cells[$i][$j + 1] == WHITE) {
                if ($i == $row_goal && $j + 1 == $col_goal) {
                    return true;
                }
                $next[] = [$i, $j + 1];
            }
            if (count($next) == 0) {
                return false;
            }
            foreach ($next as $n) {
                $result = $this->find_route($n[0], $n[1]);
                if ($result === true) {
                    return true;
                }
            }
            return false;
        }    

        private function get_another_color($v) {
            return $v == WHITE ? BLACK : WHITE;
        }
    }
    
    /**
    * お手伝い関数さんたち
    */
    function in() {
        return trim(fgets(STDIN));
    }
    function in_int() {
        return (int)in();
    }
    function in_vals() {
        return explode(' ', in());
    }
    function in_ints() {
        return array_map('intval', in_vals());
    }
    function out($s) {
        echo $s . PHP_EOL;
    }
    function yes() {
        out('Yes');
    }
    function no() {
        out('No');
    }