# coding: utf-8
# Your code here!

import sys
sys.setrecursionlimit(10**6)
readline = sys.stdin.readline 

#n = int(input())
h,w = [int(i) for i in readline().split()]
b = [input() for _ in range(h)]

def yoko(x):
    bx = b[x]
    c = bx[0]
    ok = 0
    for i in range(w):
        if bx[i] != c: ok = 1
    return ok

def tate(x):
    c = b[0][x]
    ok = 0
    for i in range(h):
        if b[i][x] != c: ok = 1
    return ok

ok = 0
if all(yoko(i) for i in range(h)): ok = 1
if all(tate(i) for i in range(w)): ok = 1

if ok: print("YES")
else: print("NO")