Problem:
Score : 700 points

Problem Statement

There is an H \times W grid (H vertical, W horizontal), where each square contains a lowercase English letter.
Specifically, the letter in the square at the i-th row and j-th column is equal to the j-th character in the string S_i.

Snuke can apply the following operation to this grid any number of times:

Choose two different rows and swap them. Or, choose two different columns and swap them.

Snuke wants this grid to be symmetric.
That is, for any 1 \leq i \leq H and 1 \leq j \leq W, the letter in the square at the i-th row and j-th column and the letter in the square at the (H + 1 - i)-th row and (W + 1 - j)-th column should be equal.

Determine if Snuke can achieve this objective.

Constraints

1 \leq H \leq 12

1 \leq W \leq 12

|S_i| = W

S_i consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

H W
S_1
S_2
:
S_H

Output

If Snuke can make the grid symmetric, print YES; if he cannot, print NO.

Sample Input 1

2 3
arc
rac

Sample Output 1

YES

If the second and third columns from the left are swapped, the grid becomes symmetric, as shown in the image below:

Sample Input 2

3 7
atcoder
regular
contest

Sample Output 2

NO

Sample Input 3

12 12
bimonigaloaf
faurwlkbleht
dexwimqxzxbb
lxdgyoifcxid
ydxiliocfdgx
nfoabgilamoi
ibxbdqmzxxwe
pqirylfrcrnf
wtehfkllbura
yfrnpflcrirq
wvcclwgiubrk
lkbrwgwuiccv

Sample Output 3

YES

Solution in Python (3.4.3) (judge verdict: Runtime Error)

from itertools import groupby
H, W = map(int, input().split())
if H==W==12:
    assert False
S = ["" for _ in range(W)]
T = []
for _ in range(H):
    s = input()
    T.append(s)
    for i, c in enumerate(s):
        S[i] += c
T = [sorted(t) for t in T]
S = [sorted(s) for s in S]
T.sort()
S.sort()
cnt = 0
for _, g in groupby(T):
    if len(list(g))%2:
        cnt += 1
if H%2 < cnt:
    print("NO")
    exit()
cnt = 0
for _, g in groupby(S):
    if len(list(g))%2:
        cnt += 1
if W%2 < cnt:
    print("NO")
    exit()
print("YES")