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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.*;
import java.util.*;

class Main{
    final static long INF = Long.MAX_VALUE / 2;
    final static int MOD = 1_000_000_007;
    final static int SIZE = 1_000_000;
    long[] fac = new long[SIZE];
    long[] inv = new long[SIZE];
    long[] finv = new long[SIZE];
    FastScanner sc = new FastScanner();

    public static void main(String[] args) {
        new Main().solve();
    }

    void solve(){

        int h = sc.nextInt();
        int w = sc.nextInt();
        char[][] m = new char[h][w];

        for(int i = 0; i < h; i++){
            m[i] = sc.next().toCharArray();
        }

        int[][] a = new int[h][w];
        for(int i = 0; i < h; i++) Arrays.fill(a[i], Integer.MAX_VALUE / 2);
        a[0][0] = m[0][0] == '#' ? 1 : 0;

        for(int i = 0; i < h; i++){
            for(int j = 0; j < w; j++){
                if(i > 0){
                    int x = a[i - 1][j];
                    if(m[i][j] == '#' && m[i - 1][j] == '.') x++;
                    a[i][j] = Math.min(a[i][j], x);
                }
                if(j > 0){
                    int x = a[i][j - 1];
                    if(m[i][j] == '#' && m[i][j - 1] == '.') x++;
                    a[i][j] = Math.min(a[i][j], x);
                }
            }
        }

        for(int i = 0; i < h; i++){
            for(int j = 0; j < w; j++){
                System.err.print(a[i][j] + " ");
            }
            System.err.println();
        }

        System.out.println(a[h - 1][w - 1]);

    }

    long gcd(long a, long b){ // return aとbの最大公約数
        if(b == 0){
          return a;
        }
        return gcd(b, a % b);
    }

    long lcm(long a, long b){ // return aとbの最小公倍数
        return a * b / gcd(a, b);
    }

    long inv(long a){ // return aの逆元 (mod MOD)
        return pow(a, MOD - 2);
    }

    long pow(long a, long r){ // return a^r (mod MOD)
        long sum = 1;
        while(r > 0){
            if((r & 1) == 1){ // 2進数表記で末尾1の時
                sum *= a;
                sum %= MOD;
            }
            a *= a;
            a %= MOD;
            r >>= 1;
        }
        return sum;
    }

    long modFact(long n){ // retur n! (mod MOD)
        if(n == 0){
            return 1;
        }
        return n * modFact(n - 1) % MOD;
    }

    long fact(long n){ // return n!
        if(n == 0){
            return 1;
        }
        return n * fact(n - 1);
    }

    void initCOMB(){
        fac[0] = fac[1] = 1;
        inv[1] = 1;
        finv[0] = finv[1] = 1;
        for(int i = 2; i < SIZE; i++){
            fac[i] = fac[i - 1] * i % MOD;
            inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
            finv[i] = finv[i - 1] * inv[i] % MOD;
        }
    }

    long modComb(int n, int r){ // return nCr (先にinitCOMB()必要)
        if(n < r || n < 0 || r < 0) return 0;
        return fac[n] * finv[r] % MOD * finv[n - r] % MOD;
    }

    long comb(long n, long r){ // return nCr
        long num = 1;
        for(long i = 1; i <= r; i++){
            num = num * (n - i + 1) / i;
        }
        return num;
    }

    boolean isPrime(long a){ // aの素数判定
        if(a <= 1) return false;
        for(int i = 2; i * i <= a; i++){
            if(a % i == 0) return false;
        }
        return true;
    }

    int lowerBound(long[] a, long v){ // return 配列a内のv以上の要素の内最低の要素のイテレータ
        int r = a.length;
        int l = -1;
        while(r - l > 1){
            int mid = (r + l) / 2;
            if(a[mid] >= v){
                r = mid;
            }else{
                l = mid;
            }
        }
        return r;
    }

    int upperBound(long[] a, long v){ // return 配列a内のvより大きい要素の内最低の要素のイテレータ
        int r = a.length;
        int l = -1;
        while(r - l > 1){
            int mid = (r + l) / 2;
            if(a[mid] > v){
                r = mid;
            }else{
                l = mid;
            }
        }
        return r;
    }

    String nextPermutation(String s){ // return sの次の順列
        ArrayList<Character> list = new ArrayList<>();
        for(int i = 0; i < s.length(); i++) list.add(s.charAt(i));

        int pivotPos = -1;
        char pivot = 0;
        for(int i = list.size() - 2; i >= 0; i--){
            if(list.get(i) < list.get(i+1)){
    			pivotPos = i;
    			pivot = list.get(i);
    			break;
    		}
    	}

        if(pivotPos == -1 && pivot == 0) return null;

        int L = pivotPos + 1;
        int R = list.size() - 1;
    	int minPos = -1;
    	char min = Character.MAX_VALUE;
    	for(int i = R; i >= L; i--){
    		if(pivot < list.get(i)){
    			if(list.get(i) < min){
    				min = list.get(i);
    				minPos = i;
    			}
    		}
    	}

    	Collections.swap(list, pivotPos, minPos);
    	Collections.sort(list.subList(L, R + 1));

    	StringBuilder sb = new StringBuilder();
    	for(int i=0; i<list.size(); i++) sb.append(list.get(i));

    	return sb.toString();
    }

    boolean nextPermutation(long[] a){
        for(int i = a.length - 1; i > 0; i--){
            if(a[i - 1] < a[i]){
                int swapIndex = find(a[i - 1], a, i, a.length - 1);
                long temp = a[swapIndex];
                a[swapIndex] = a[i - 1];
                a[i - 1] = temp;
                Arrays.sort(a, i, a.length);
                return true;
            }
        }
        return false;
    }

    int find(long dest, long[] a, int s, int e){
        if(s == e){
            return s;
        }
        int m = (s + e + 1) / 2;
        return a[m] <= dest ? find(dest, a, s, m - 1) : find(dest, a, m, e);
    }

    void elimination(int[][] a, int[] b) {
        int n = a.length;
        double f;
        for(int k = 0; k < n - 1; k++){
            for(int i = k + 1; i < n; i++){
                f = - a[i][k] / a[k][k];
                for(int j = k + 1; j < n; j++){
                    a[i][j] += f * a[k][j];
                }
                b[i] += f * b[k];
            }
            for(int i = n - 1; i >= 0; i--){
                for(int j = i + 1; j < n; j++){
                    b[i] -= a[i][j] * b[j];
                }
                b[i] = b[i] / a[i][i];
            }
        }
    }



    class SegmentTree{

        //------------------------------------------------------------
        // 2 * n - 1 : 木全体のノード数
        // i + n - 1 : 配列のi番目が対応するノードの番号
        // 2 * i + 1, 2 * i + 2 : i番目のノードの子ノードの番号
        // (i - 1) / 2 : i番目のノードの親ノードの番号
        //
        // int n = sc.nextInt();
        // long[] a = new long[n];
        // for(int i = 0; i < n; i++) a[i] = sc.nextLong();
        // SegmentTree st = new SegmentTree(a);
        // int l = sc.nextInt() - 1;
        // int r = sc.nextInt() - 1;
        // System.out.println(st.query(l, r));
        //------------------------------------------------------------

        final static long INF = Long.MAX_VALUE / 2;

        // long e = INF; // 単位元
        long e = 0;
        long func(long a, long b){ // 処理
            // return Math.min(a, b);
            return a + b;
        }

        int n; // 配列の要素数を超える最小の2のべき乗
        long[] node;

        SegmentTree(long[] a){
            init(a);
        }

        void init(long[] a){ // 配列aで初期化
            n = 1;
            while(n < a.length){
                n *= 2;
            }
            node = new long[2 * n - 1];
            Arrays.fill(node, e);
            for(int i = 0; i < a.length; i++){
                node[i + n - 1] = a[i];
            }
            for(int i = n - 2; i >= 0; i--){
                node[i] = func(node[2 * i + 1], node[2 * i + 2]);
            }
        }

        void update(int p, long v){ // 配列のp番目をvに変更し、木全体を更新
            p = p + n - 1;
            node[p] = v;
            while(p > 0){
                p = (p - 1) / 2;
                node[p] = func(node[2 * p + 1], node[2 * p + 2]);
            }
        }

        long query(int a, int b){ // 区間[a, b)についてクエリを処理
            return query(a, b, 0, 0, n);
        }

        long query(int a, int b, int k, int l, int r){
            if(r <= a || b <= l) return e;
            if(a <= l && r <= b) return node[k];
            return func(query(a, b, 2 * k + 1, l, (l + r) / 2), query(a, b, 2 * k + 2, (l + r) / 2, r));
        }

    }



    class UnionFindTree{

        //------------------------------------------------------------
        // int n = sc.nextInt();
        // int q = sc.nextInt();
        // UnionFindTree uft = new UnionFindTree(n);
        // List<String> ans = new ArrayList<>();
        // for(int i = 0; i < q; i++){
        //     int p = sc.nextInt(); // 0 : union, 1 : same
        //     if(p == 0){
        //         int a = sc.nextInt() - 1;
        //         int b = sc.nextInt() - 1;
        //         uft.union(a, b);
        //     }else if(p == 1){
        //         int a = sc.nextInt() - 1;
        //         int b = sc.nextInt() - 1;
        //         if(uft.same(a, b)) ans.add("Yes");
        //         else ans.add("No");
        //     }
        // }
        // for(String s : ans){
        //     System.out.println(s);
        // }
        //------------------------------------------------------------

        int[] parent; // インデックスにとノードを対応させ、そのルートノードのインデックスを格納
        int[] rank; // parentと同様に、木の高さを格納

        UnionFindTree(int size){
            parent = new int[size];
            rank = new int[size];

            for(int i = 0; i < size; i++){
                makeSet(i);
            }
        }

        void makeSet(int i){
            parent[i] = i;
            rank[i] = 0;
        }

        void union(int x, int y){
            int xRoot = find(x);
            int yRoot = find(y);

            if(rank[xRoot] > rank[yRoot]){ // xが属する木の方が大きい場合
                parent[yRoot] = xRoot;
            }else if(rank[xRoot] < rank[yRoot]){
                parent[xRoot] = yRoot; // yの親をxに更新
            }else{
                parent[yRoot] = xRoot;
                rank[xRoot]++; // 同じ高さの木がルートの子として着くから大きさ++;
            }
        }

        int find(int i){ // iの属するルートを返す
            if(i != parent[i]){
                parent[i] = find(parent[i]);
            }
            return parent[i];
        }

        boolean same(int x, int y){ // xとyが同じ木に属しているかを返す
            return find(x) == find(y);
        }

    }



}



class Pair implements Comparable<Pair>{
    long x, y;
    public Pair(long i, long j){
        x = i;
        y = j;
    }

    @Override
    public int compareTo(Pair p){
        if(this.y < p.y) return -1;
        else if(this.y > p.y) return 1;
        else return 0;
    }

    @Override
    public String toString(){
        return x + " " + y;
    }
}



class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte(){
        if(ptr < buflen){
            return true;
        }else{
            ptr = 0;
            try{
                buflen = in.read(buffer);
            }catch(IOException e){
                e.printStackTrace();
            }
            if(buflen <= 0){
                return false;
            }
        }
        return true;
    }
    private int readByte(){
        if(hasNextByte()) return buffer[ptr++];
        else return -1;
    }
    private static boolean isPrintableChar(int c){
        return 33 <= c && c <= 126;
    }
    public boolean hasNext(){
        while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    }
    public String next(){
        if(!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)){
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong(){
        if(!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if(b == '-'){
            minus = true;
            b = readByte();
        }
        if(b < '0' || '9' < b){
            throw new NumberFormatException();
        }
        while(true){
            if('0' <= b && b <= '9'){
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt(){
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble(){
        return Double.parseDouble(next());
    }
}