import sys
H, W = map(int, input().split())
a = [list(map(int, l.rstrip())) for l in sys.stdin]

if all(len(set(l)) == 2 for l in a):
    print('YES')
    exit()
for x in range(W):
    v = [0, 0]
    for y in range(H):
        v[a[y][x]] = 1
    if v[0] == 0 or v[1] == 0:
        print('NO')
        exit()
print('YES')