Nguồn:
http://codeforces.com
You
play a computer game. Your character stands on some level of a multilevel ice
cave. In order to move on forward, you need to descend one level lower and the
only way to do this is to fall through the ice.
The
level of the cave where you are is a rectangular square grid of n rows
and m columns. Each cell consists either from intact or from
cracked ice. From each cell you can move to cells that are side-adjacent with
yours (due to some limitations of the game engine you cannot make jumps on the
same place, i.e. jump from a cell to itself). If you move to the cell with
cracked ice, then your character falls down through it and if you move to the
cell with intact ice, then the ice on this cell becomes cracked.
Let's
number the rows with integers from 1 to n from top
to bottom and the columns with integers from 1 to m from
left to right. Let's denote a cell on the intersection of the r-th
row and the c-th column as (r, c).
You
are staying in the cell (r1, c1) and
this cell is cracked because you've just fallen here from a higher level. You
need to fall down through the cell (r2, c2) since
the exit to the next level is there. Can you do this?
Input
The
first line contains two integers, n and m (1 ≤ n, m ≤ 500) —
the number of rows and columns in the cave description.
Each
of the next n lines describes the initial state of the level
of the cave, each line consists of m characters "."
(that is, intact ice) and "X" (cracked ice).
The
next line contains two integers, r1 and c1 (1 ≤ r1 ≤ n, 1 ≤ c1 ≤ m) —
your initial coordinates. It is guaranteed that the description of the cave
contains character 'X' in cell (r1, c1),
that is, the ice on the starting cell is initially cracked.
The
next line contains two integers r2 and c2 (1 ≤ r2 ≤ n, 1 ≤ c2 ≤ m) —
the coordinates of the cell through which you need to fall. The final cell may
coincide with the starting one.
Output
If
you can reach the destination, print 'YES', otherwise print 'NO'.
Sample
Input
|
Input
|
Output
|
|
Input
|
Output
|
|
Input
|
Output
|
|
4
6
X...XX
...XX. .X..X. ...... 1 6
2
2
|
YES
|
|
5
4
.X..
...X X.X. .... .XX. 5 3
1
1
|
NO
|
|
4
7
..X.XX.
.XX..X. X...X.. X...... 2 2
1
6
|
YES
|
Hint
In
the first sample test one possible path is:
After
the first visit of cell (2, 2) the ice on it cracks and when you step
there for the second time, your character falls through the ice as intended.
Tóm tắt đề
Cho một bảng kích thước N x M. Những ô ‘.’ là những ô băng chưa nứt.
Những ô ‘X’ là những ô băng đã nứt. Nếu nhảy lên ô ‘.’ thì băng từ chưa nứt
thành băng đã nứt. Còn nếu nhảy lên ô ‘X’ thì băng từ đã nứt thành sụp luôn.
Yêu cầu từ ô (r1 c1) hỏi có đường để nhảy đến ô (r2 c2) hay không ? Nhưng mà
khi tới được ô (r2 c2) rồi, thì phải nhảy làm sao cho ô (r2 c2) sụp luôn thì
mình thắng. Ngược lại thì thua.
