#!/usr/bin/ python3.8
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import numpy as np

H, W = map(int, readline().split())
A = np.frombuffer(read(), 'S1').reshape(H, -1)[:, :W].astype(int)


def solve(A):
    H, W = A.shape
    if H == 1 and W == 1:
        return A[0, 0] == 1
    is_monotone_col = np.isin(np.sum(A, axis=0), [0, H])
    is_monotone_row = np.isin(np.sum(A, axis=1), [0, W])
    return not (np.any(is_monotone_col) and np.any(is_monotone_row))


print('YES' if solve(A) else 'NO')