def solve():
    N=int(input())
    M=int(input())

    T=[list(input()[:-1]) for _ in range(M)]
    T=["".join(list(t)) for t in zip(*T)]
    return len(set(T))==N

#==================================================
import sys
input=sys.stdin.readline
write=sys.stdout.write

print("Yes" if solve() else "No")