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 Fortran (gfortran v4.8.4) (judge verdict: Wrong Answer)

program main
    implicit none
    integer(4):: i, j, k
    integer(4):: h, w, cnt=0
    integer(4), parameter:: inf = 1000000
    integer(4), allocatable:: m(:,:), im(:,:), tmp(:,:), dp(:,:)
    character(100)::pcm
    character(1),allocatable:: cm(:,:)
    logical, allocatable:: check(:,:)
    logical:: flip

    read*, h, w
    allocate(cm(w,h), m(w,h), im(w,h), dp(w,h), check(w,h))

    do i=1, h
        read*, pcm
        do j=1, w
            if (pcm(j:j) == '.') then
                m(j,i) = 1
            else
                m(j,i) = 0
            end if
        end do
    end do

    dp(:,:) = inf

    flip = .false.
    call init(m(:,:),1,1, flip)

    dp(1,1) = 0
    if (flip) dp(1,1) = 1
    ! do i=1,h
    !     print*, m(:,i)
    ! end do

    do while(dp(w,h) >= inf)
        check(:,:) = .false.
        do i=1, h
            do j=1, w
                if (check(j,i) .or. dp(j,i) == inf) cycle

                if (m(j,i) == m(j,i+1) .and. i+1 <= h) then
                    dp(j, i+1) = dp(j, i)
                else if (i+1 <= h) then
                    dp(j,i+1) = min(dp(j,i)+1, dp(j,i+1))
                    check(j,i+1) = .true.
                end if

                if (m(j,i) == m(j+1, i) .and. j+1 <= w) then
                    dp(j+1, i) = dp(j, i)
                else if (j+1 <= w) then
                    dp(j+1,i) = min(dp(j,i)+1, dp(j+1,i))
                    check(j+1,i) = .true.
                end if
                ! print*, '-'
                ! print*, m(j,i) == m(j,i+1), i+1 <= h
                ! print*, m(j,i) == m(j+1, i), j+1 <= w
                ! do k=1, h
                !     print*, dp(:,k)
                ! end do
            end do
        end do
    !     print*, '-- --'

    ! do i=1, h
    !     print*, dp(:,i)
    ! end do

    ! do i=1, h
    !     print*, check(:,i)
    ! end do

        do i=1, h
            do j=1, w
                if (.not. check(j,i)) cycle

                if (i+1 <= h) dp(j,i+1) = min(dp(j,i), dp(j,i+1))
                if (j+1 <= w) dp(j+1,i) = min(dp(j,i), dp(j+1,i))
            end do
        end do

        ! print*, '---'
        ! do i=1,h
        !     print*, dp(:,i)
        ! end do

    end do
    print*, dp(w,h)

contains
recursive subroutine init(m, j, i, f)
    integer(4):: m(:,:),i,j
    logical:: f
    if (m(j,i) == 0) then
        f = .true.
        if (i+1 <= h) call init(m(:,:), j, i+1, f) 
        if (j+1 <= w) call init(m(:,:), j+1, i, f)
        m(j,i) = 1
    end if 
    
end subroutine init
    
end program main