Range Flip Find Route

AtCoder

IDagc043_a

Time2000ms

Memory256MB

Difficulty—

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. [samples]

Samples

Input #1

3 3
.##
.#.
##.

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.

Input #2

2 2
#.
.#

Output #2

Input #3

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

Output #3

0

No operation may be needed.

Input #4

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

Output #4

API Response (JSON)

{
  "problem": {
    "name": "Range Flip Find Route",
    "description": {
      "content": "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. Th",
      "description_type": "Markdown"
    },
    "platform": "AtCoder",
    "limit": {
      "time_limit": 2000,
      "memory_limit": 262144
    },
    "difficulty": "None",
    "is_remote": true,
    "is_sync": true,
    "sync_url": null,
    "sign": "agc043_a"
  },
  "statements": [
    {
      "statement_type": "Markdown",
      "content": "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.\nTh...",
      "is_translate": false,
      "language": "English"
    }
  ]
}

Full JSON Raw Segments