N,M=map(int,input().split())
A=[list(map(int,list(input()))) for i in range(N)]
blN,blM=False,False
for i in range(N):
    if len({A[i][j] for j in range(M)})==1:
        blN=True
for j in range(M):
    if len({A[i][j] for i in range(N)})==1:
        blM=True
if blN and blM:
    ans="NO"
else:
    ans="YES"
print(ans)