N=int(input())
M=int(input())
S=[input() for i in range(M)]

LIST=[0]*(N*(N-1)//2)

for i in range(M):
    x=S[i]
    now=0

    for j in range(N):
        for k in range(j+1,N):
            if x[j]!=x[k]:
                LIST[now]=1

            now+=1

if N==1 or min(LIST)==1:
    print("Yes")
else:
    print("No")