h, w = map(int, input().split())
f = [[int(c) for c in input()] for _ in range(h)]

yes = 2

for i in range(h):
    same = True
    for j in range(w):
        same = same and f[i][j] == f[i][0]
    if same and yes == 2: yes = 1

for j in range(w):
    same = True
    for i in range(h):
        same = same and f[i][j] == f[0][j]
    if same and yes == 1: yes = 0

print("YES" if yes > 0 else "NO")