import sys
import random

M = int(input())
N = int(input())

S = [input() for i in range(N)]
s = set()

for j in range(M):
    
    col = "".join( [S[i][j] for i in range(N)] )
    s.add(col)

if len(s) != M:
    print ("No")
else:
    print ("Yes")