H,W=map(int,input().split())
a=[]
for i in range(H):
 a.append(list(map(int,input().rstrip())))
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')