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: Runtime Error)

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

namespace ClassLibrary1
{
    public class ListNode
    {
        public int val;
        public ListNode next;
        public ListNode(int x) { val = x; }
    }


    public class Class1
    {
        //static Dictionary<Tuple<long, long>, long> dic = new Dictionary<Tuple<long, long>, long>();

        static long mod = 1000000007;
        static long[] factorial = new long[1000001];
        static long[] invFactorial = new long[1000001];

        private static void calcFanctorial(long N)
        {
            factorial[0] = 1;
            for (int i = 1; i < N + 1; i++)
            {
                factorial[i] = factorial[i - 1] * i;
                factorial[i] %= mod;
            }
            invFactorial[N] = inv(factorial[N]);
            for (int i = (int)N; i > 0; i--)
            {
                invFactorial[i - 1] = invFactorial[i] * i % mod;
            }

        }

        static short[][] mem;


        static void Main(string[] args)
        {
            var N = ReadLongArr();

            var mtx = ReadCharMatrix(N[0]);
            mem = new short[N[0]][];
            for (int i = 0; i < N[1]; i++)
            {
                mem[i] = new short[N[1]];
            }

            /*
            mem[0][0] = (short)(mtx[0][0] == '.' ? 0 : 1);

            for (int i = 0; i < N[0]; i++)
            {
                for (int j = 0; j < N[1]; j++)
                {
                    if (i == 0 && j == 0) continue;
                    if (i == 0)
                    {
                        var pp = (mtx[i][j - 1] != mtx[i][j] && mtx[i][j] != '.') ? 1 : 0;
                        mem[i][j] = (short)(mem[i][j - 1] + pp);
                    }
                    else if (j == 0)
                    {
                        var pp = (mtx[i-1][j] != mtx[i][j] && mtx[i][j] != '.') ? 1 : 0;
                        mem[i][j] = (short)(mem[i-1][j] + pp);
                    }
                    else
                    {
                        var a = mem[i][j - 1] + (mtx[i][j-1] != mtx[i][j] &&  mtx[i][j] != '.' ? 1 : 0);
                        var b = mem[i - 1][j] + (mtx[i - 1][j] != mtx[i][j] && mtx[i][j] != '.' ? 1 : 0);
                        mem[i][j] = (short)(Math.Min(a, b));
                    }
                }
            }
            */


            //Console.WriteLine(mem[N[0]-1][N[1]-1]);

        }



        class Tree
        {
            public HashSet<long> conn = new HashSet<long>();
        }


        static long bin(long a, long x, long n)
        {
            long y = a;
            long res = 1;
            while (x > 0)
            {
                if (x % 2 == 1)
                {
                    res = res * y % n;
                }
                x /= 2;
                y = y * y % n;
            }
            return res;
        }

        int cmp(string a, string b)
        {
            for (var i = 0; i < Math.Min(a.Length, b.Length); i++)
                if (a[i] < b[i]) return -1;
                else if (a[i] > b[i]) return 1;
            if (a.Length == b.Length) return 0;
            if (a.Length > b.Length) return 1;
            else return -1;
        }

        private static long nCm(long n, long m)
        {
            if (n < m) return 0;
            if (m < 0) return 0;

            return (factorial[n] * invFactorial[m] % mod) * invFactorial[n - m] % mod;
        }

        private static long nCm2(long n, long m)
        {
            if (n < m) return 0;
            if (m < 0) return 0;

            long res = 1;
            for (long i = n; i > n - m; i--)
            {
                res *= i;
                res %= mod;
            }

            return res * invFactorial[m] % mod;
        }

        public static long pow(long a, long b, long mod)
        {
            a %= mod;

            if (b <= 0) return 1;
            var t = pow(a, b / 2, mod);
            if ((b & 1) == 0) return t * t % mod;
            return t * t % mod * a % mod;
        }

        public static long Lcm(long a, long b)
        {
            return a * b % mod * inv(Gcd(a, b)) % mod;
        }

        public static long Lcm(long[] a)
        {
            List<long> vect = new List<long>();
            long lcm = 1;
            for (int i = 0; i < a.Count(); i++)
            {
                var factor = a[i] / GcdVector(vect, a[i]);
                vect.Add(factor);
                lcm = lcm * factor % mod;
            }
            return lcm;
        }

        static long GcdVector(List<long> a, long b)
        {
            long ma = 1;
            for (int i = 0; i < a.Count; i++)
                ma = ma * a[i] % b;
            return Gcd(b, ma);
        }

        public static long Gcd(long a, long b)
        {
            if (a < b)
                // 引数を入替えて自分を呼び出す
                return Gcd(b, a);
            while (b != 0)
            {
                var remainder = a % b;
                a = b;
                b = remainder;
            }
            return a;
        }

        public static long pow(long a, long b) => pow(a, b, mod);
        public static long inv(long a) => pow(a, mod - 2);

        private static char[][] ReadCharMatrix(long h)
        {
            char[][] mtx = new char[h][];
            for (int i = 0; i < h; i++)
            {
                var s = ReadString();
                mtx[i] = s.ToCharArray();
            }
            return mtx;
        }

        private static int[][] CreateIntMatrix(long h, long w)
        {
            int[][] mtx = new int[h][];
            for (int i = 0; i < h; i++)
            {
                mtx[i] = new int[w];
            }
            return mtx;
        }


        private static string ReadString()
        {
            return Console.ReadLine();
        }
        private static string[] ReadStringArr()
        {
            return Console.ReadLine().Split(' ');
        }
        private static long ReadLong()
        {
            return long.Parse(Console.ReadLine());
        }
        private static long[] ReadLongArr()
        {
            string[] s = ReadStringArr();
            long[] l = new long[s.Length];
            for (int i = 0; i < l.Length; i++)
            {
                l[i] = long.Parse(s[i]);
            }

            return l;
        }
    }
    class StringComparator2 : IComparer<string>
    {
        int IComparer<string>.Compare(string a, string b)
        {
            int length = Math.Min(a.Length, b.Length);
            //Console.WriteLine("Comparing with length {0}", length);
            for (int i = 0; i < length; i++)
            {
                if (a[i] < b[i])
                {
                    //Console.WriteLine("returning -1");
                    return -1;
                }
                else if (a[i] > b[i]) return 1;
            }
            if (a.Length < b.Length) return -1;
            if (a.Length > b.Length) return 1;

            //Console.WriteLine("Why are we at the end?");
            return 0;
        }
    }

    public class PriorityQueue<T> where T : IComparable
    {
        private IComparer<T> _comparer = null;
        private int _type = 0;

        private T[] _heap;
        private int _sz = 0;

        private int _count = 0;

        /// <summary>
        /// Priority Queue with custom comparer
        /// </summary>
        public PriorityQueue(int maxSize, IComparer<T> comparer)
        {
            _heap = new T[maxSize];
            _comparer = comparer;
        }

        /// <summary>
        /// Priority queue
        /// </summary>
        /// <param name="maxSize">max size</param>
        /// <param name="type">0: asc, 1:desc</param>
        public PriorityQueue(int maxSize, int type = 0)
        {
            _heap = new T[maxSize];
            _type = type;
        }

        private int Compare(T x, T y)
        {
            if (_comparer != null) return _comparer.Compare(x, y);
            return _type == 0 ? x.CompareTo(y) : y.CompareTo(x);
        }

        public void Push(T x)
        {
            _count++;

            //node number
            var i = _sz++;

            while (i > 0)
            {
                //parent node number
                var p = (i - 1) / 2;

                if (Compare(_heap[p], x) <= 0) break;

                _heap[i] = _heap[p];
                i = p;
            }

            _heap[i] = x;
        }

        public T Pop()
        {
            _count--;

            T ret = _heap[0];
            T x = _heap[--_sz];

            int i = 0;
            while (i * 2 + 1 < _sz)
            {
                //children
                int a = i * 2 + 1;
                int b = i * 2 + 2;

                if (b < _sz && Compare(_heap[b], _heap[a]) < 0) a = b;

                if (Compare(_heap[a], x) >= 0) break;

                _heap[i] = _heap[a];
                i = a;
            }

            _heap[i] = x;

            return ret;
        }

        public int Count()
        {
            return _count;
        }

        public T Peek()
        {
            return _heap[0];
        }

        public bool Contains(T x)
        {
            for (int i = 0; i < _sz; i++) if (x.Equals(_heap[i])) return true;
            return false;
        }

        public void Clear()
        {
            while (this.Count() > 0) this.Pop();
        }

        public IEnumerator<T> GetEnumerator()
        {
            var ret = new List<T>();

            while (this.Count() > 0)
            {
                ret.Add(this.Pop());
            }

            foreach (var r in ret)
            {
                this.Push(r);
                yield return r;
            }
        }

        public T[] ToArray()
        {
            T[] array = new T[_sz];
            int i = 0;

            foreach (var r in this)
            {
                array[i++] = r;
            }

            return array;
        }
    }


}