import numpy as np
h, w = map(int, input().split())
s = [list(input()) for _ in range(h)]
s = np.array(s)
out = "Yes"
for i in range(h):
    for j in range(w):
        if i == 0 and j == 0:
            up = s[i][j]
            down = s[i + 1][j]
            left = s[i][j]
            right = s[i][j + 1]
        elif i == 0 and j == w-1:
            up = s[i][j]
            down = s[i + 1][j]
            left = s[i][j - 1]
            right = s[i][j]
        elif i == h-1 and j == 0:
            up = s[i - 1][j]
            down = s[i][j]
            left = s[i][j]
            right = s[i][j + 1]
        elif i == h-1 and j == w-1:
            up = s[i - 1][j]
            down = s[i][j]
            left = s[i][j - 1]
            right = s[i][j]
        elif i == 0 and j != 0 and j != w-1:
            up = s[i][j]
            down = s[i + 1][j]
            left = s[i][j - 1]
            right = s[i][j + 1]
        elif i == h-1 and j != 0 and j != w-1:
            up = s[i - 1][j]
            down = s[i][j]
            left = s[i][j - 1]
            right = s[i][j + 1]
        elif j == 0 and i !=  0 and i != h-1:
            up = s[i - 1][j]
            down = s[i + 1][j]
            left = s[i][j]
            right = s[i][j + 1]
        elif j == w-1 and i != 0 and i!= h-1:
            up = s[i - 1][j]
            down = s[i + 1][j]
            left = s[i][j - 1]
            right = s[i][j]
        else:
            up = s[i-1][j]
            down = s[i+1][j]
            left = s[i][j-1]
            right = s[i][j+1]
        if s[i][j] != up and s[i][j] != down and s[i][j] != left and s[i][j] != right:
            out = "No"
print(out)