n = int(input())
m = int(input())
s_list = [input().strip() for _ in range(m)]

# Reverse each string to get the rightmost character as the first element
s_reversed = [s[::-1] for s in s_list]

# Transpose the reversed strings to get columns
columns = [''.join(col) for col in zip(*s_reversed)]

# Check if all columns are unique
if len(columns) == len(set(columns)):
    print("Yes")
else:
    print("No")