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

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


class Program {

	static int testN = 3;
	static long mod = 1000000007;
	//static long mod =998244353; 
	static System.IO.StringReader sr;
		static void Main(string[] args) {
			char[] cs = new char[] { ' ' };
			//char[] cs = new char[] { ' ', ',' };

			//int n = int.Parse(Console.ReadLine());
			//int val = int.Parse(Console.ReadLine());
			//int[] vals = parseAry(Console.ReadLine().Split(cs));
			//string str = Console.ReadLine();
			//string[] strs = Console.ReadLine().Split(cs);

			//string res = "";
			int res = -1;

			string[] rl = Console.ReadLine().Split(cs);
			int ylen = int.Parse(rl[0]);
			int xlen = int.Parse(rl[1]);


			string[] map = new string[ylen];
			for ( int i = 0; i < ylen; i++ ) {
				map[i] = Console.ReadLine();
			}



			int[] dx = { 1, 0 };
			int[] dy = { 0, 1 };
			int[,] count = new int[ylen, xlen];	//[x,y]＝#通過回数

			for ( int i = 0; i < ylen; i++ ) {
				for ( int k = 0; k < xlen; k++ ) {
					count[i, k] = int.MaxValue - 10;
				}
			}

			Queue<Point> qu = new Queue<Point>();

			count[0, 0] = 0;	//初期値マス
			if ( map[0][0] == '#' ) {
				count[0, 0] = 1;
				qu.Enqueue(new Point(1, 0, 0, 0));//最初
			} else {
				qu.Enqueue(new Point(0, 0, 0, 0));//最初
			}

			while ( qu.Count > 0 ) {

				Point temp = qu.Dequeue();
				int nowCnt = (int)count[temp.y, temp.x];
				for ( int i = 0; i < dx.Length; i++ ) {	//2方向のマスを調べる
					int tx = temp.x + dx[i];
					int ty = temp.y + dy[i];
					if ( tx < 0 || xlen <= tx ) { continue; }
					if ( ty < 0 || ylen <= ty ) { continue; }
					if ( map[ty][tx] == '.' ) {
						if ( nowCnt >= count[ty, tx] ) { continue; } else {
							count[ty, tx] = nowCnt;
							qu.Enqueue(new Point(0, tx, ty, nowCnt));
							continue;
						}

					}
					if ( map[ty][tx] == '#' ) {
						if ( temp.type == 0 ) {
							if ( nowCnt + 1 >= count[ty, tx] ) { continue; } else {
								count[ty, tx] = nowCnt + 1;
								qu.Enqueue(new Point(1, tx, ty, nowCnt + 1));
								continue;
							}
						} else if ( temp.type ==1 ) {
							if ( nowCnt >= count[ty, tx] ) { continue; } else {
								count[ty, tx] = nowCnt;
								qu.Enqueue(new Point(1, tx, ty, nowCnt + 1));
								continue;
							}
						
						}

					}
				}
			}
			int min = int.MaxValue;
			for ( int i = 0; i < ylen; i++ ) {
				for ( int k = 0; k < xlen; k++ ) {
					min=Math.Min(count[i, k],min);
				}
			}
			Console.WriteLine(count[ylen-1,xlen-1]);
			return;

		
	}

		struct Point {
			public int type;
			public int x;
			public int y;
			public int mon;
			public Point(int a, int b, int c, int d) {
				type = a;
				x = b;
				y = c;
				mon = d;
			}
		}

	//	文字列strをindex番目からdで上書き
	static string strOverWrite(string str, int i, string s) {
		return str.Remove(i, s.Length).Insert(i, s);
	}

	//　String[]  ----->  int[]
	static int[] parseAry(string[] str) {
		int[] nums = new int[str.Length];
		for ( int i = 0; i < str.Length; i++ ) { nums[i] = int.Parse(str[i]); }
		return nums;
	}
	//　String[]  ----->  long[]
	static long[] parseAryL(string[] str) {
		long[] nums = new long[str.Length];
		for ( int i = 0; i < str.Length; i++ ) { nums[i] = long.Parse(str[i]); }
		return nums;
	}

	//文字列を1文字ずつString[]に入れる
	static string[] strAry(String str) {
		string[] re = new string[str.Length];
		for ( int i = 0; i < str.Length; i++ ) { re[i] = str[i].ToString(); }
		return re;
	}



}